The species name is a patronym, in genitive plural, created in honour of Clifford H. Pope (1899–1974) and his wife Sarah H. Pope (1901–1995); see below for a discussion of the correct spelling. We recommend the following common names for this species: “Pō pǔ zhú yè qīng” (坡普竹叶青) (in Chinese), “Pope’s green pitviper” (in English), “Ngu Khiew Hang Mai Thong Khiew Assam” (งูเขียวหางไหม้ท้องเขียวอัสสัม) (in Thai), “Trimérésure vert des Pope” (in French), “Popes Bambusotter” (in German), and “Bambukovaya kufiya Poupov” (in Russian).

Before Smith (1937), the species Trimeresurus popeiorum s. str. was widely mentioned in the literature under the combinations T. gramineus or Lachesis gramineus (non Coluber gramineus Shaw, 1802, a distinct, valid species from Peninsular India; see, for example, Pope and Pope 1933: 9). As explained above, Regenass and Kramer (1981) divided this species into three subspecies, retaining the nominative one, T. p. popeiorum, for populations of the Asian mainland. Currently, following Vogel et al. (2004), Wostl et al. (2016), and the present work, Trimeresurus popeiorum s. str. is monotypic.

We take this opportunity to mention the taxon Coluber viridicaeruleus La Cépède, 1789, which Wallach et al. (2014: 575) placed in the synonymy of T. popeiorum. La Cepède (1789: 122 and 306) erected this taxon as a replacement name for Coluber cyaneus Linnaeus, 1758. Based on its scalation characters, such as a head covered with large plates and 110 subcaudals, this taxon is obviously not a pitviper, but may represent a nearctic or neotropical colubrid species, as it was stated to have originated from “Les Amériques”. Therefore, we formally remove Coluber viridicaeruleus from the synonymy of Trimeresurus popeiorum, while the taxonomic identity of Coluber viridicaeruleus La Cépède, 1789 remains unclear.

A nomenclatural problem arose when Smith (1943: 518) stated in a footnote that the spelling popeiorum in his earlier paper was a clerical error and, on the same page, corrected it into popeorum. As a result, the spelling popeorum has been widely used in the literature, for example by Haas (1950), Taylor and Elbel (1958), Klemmer (1963), Werler and Keegan (1963), Taylor (1965), Leviton (1968), Harding and Welch (1980), and Regenass and Kramer (1981). David and Vogel (1996: 163) were the first authors to discuss the spelling problem and pointed out that, based on Art. 32 (c) (ii) of the Code (ICZN 1985), the original spelling had to be retained even though Smith (1943) tried to correct the original spelling. According to Art. 32.5.1 of the Code (ICZN 1999), the initial spelling was an incorrect transliteration or Latinization and does not allow for “correction of an incorrect original spelling”. According to the Art. 33.4, the use of a termination -orum in a subsequent spelling of a species-group name that is a genitive based upon a personal name in which the correct original spelling terminates with -iorum, is an incorrect subsequent spelling, even if the change is deliberate. The original spelling, popeiorum, must therefore be conserved. McDiarmid et al. (1999) and Orlov et al. (2002a, 2002b) challenged this interpretation and argued that the spelling popeorum was the correct one. However, these authors have not been followed.

This taxon has long been forgotten or considered a synonym of Trimeresurus gramineus. For example, Uetz et al. (2024) currently list this taxon in the synonymy of Craspedocephalus gramineus (Shaw, 1802) based on Stejneger (1907, 1927). These authors apparently overlooked the fact that Stejeneger confused several green pitvipers under the combination Trimeresurus gramineus. The type locality of Trimesurus elegans makes clear that it cannot be a synonym of T. gramineus, as currently defined, a species endemic to southeastern Peninsular India (see Mallik et al. 2021).

The brief original description of Trimesurus elegans does not allow a formal identification of the species. However, its holotype (NHMUK 1946.1.19.20), which we examined, is indeed a subadult female referable to Trimeresurus popeiorum s. str. as defined here. On this basis, Trimesurus elegans Gray, 1853 should be considered a senior subjective synonym of T. popeiorum. Such a conclusion would be problematic in terms of nomenclatural stability, as the species name elegans has seemingly never been used as valid after its establishment by Gray (1853), while the popeiorum has been used as valid in dozens of publications since 1937 in the genera Trimeresurus or Popeia.

For this reason, in order to protect the binomen Trimeresurus popeiorum Smith, 1937, we here make use of Art. 23.9 of the Code to reverse precedence (ICZN 1999). In order to apply this Article, two conditions must be fulfilled: Art. 23.9.1.1 states that the senior synonym must not have been used as a valid nomen after 1899; and Article 23.9.1.2 states that the junior synonym must have been used for a particular taxon, as its presumed valid nomen, “in at least 25 works, published by at least 10 authors in the immediately preceding 50 years and encompassing a span of not less than 10 years” (ICZN 1999).

In the present case, the first condition is met because to the best of our knowledge, the specific epithet elegans, as proposed by Gray (1853), has never been used as valid for any pitviper after 1853. It should be noted that Craspedocephalus elegans Gray, 1849, now Protobothrops elegans, a valid species inhabiting Japan, was placed in the genus Trimeresurus by Stejneger (1907). In this case, Trimesurus elegans Gray, 1853 also becomes a secondary homonym of Craspedocephalus elegans Gray, 1849 in the genus Trimeresurus.

In order to fulfil the second condition, the epithet popeiorum should have been used as valid in the genus Trimeresurus, even if under the erroneous spelling popeorum, by at least 25 authors since 1972. We provide below a list of 25 works published from 1972 onwards in which the epithet popeiorum (or popeorum) has been used as valid: Harding and Welch (1980: 74); Regenass and Kramer (1981: 186; as T. p. popeorum); Tweedie (1983: 139, 158); Toriba in Golay et al. (1993: 103; as T. popeorum); David and Ineich (1999: 288); Leviton et al. (2003: 446); Gumprecht et al. (2004: 37, 256; as Trimeresurus popeiorum popeiorum); Malhotra and Thorpe (2004: 97); Sanders et al. (2004: 183); Vogel et al. (2004: 19); Castoe and Parkinson (2006: 105); Sanders et al. (2006: 361); Vogel (2006: 107); Leviton et al. (2008: 80); Das (2010: 307); Malhotra et al. (2010: 175); Teynié and David (2010: 268); Wallach et al. (2014: 575); Visser (2015: 387); Guo et al. (2015: 267); Chan-ard et al. (2015: 345); Das et al. (2016: 275-276); Wostl et al. (2016: 637); Chen et al. (2019: 20); Nguyen et al. (2020: 239); Liu et al. (2022: 86); and Rathee et al. (2022: 3)

The foregoing sources allow to conclude that Art. 23.9 can be applied to Trimesurus elegans and precedence can be reversed. We therefore declare Trimesurus elegans Gray, 1853 to be a subjective senior synonym of Trimeresurus popeiorum Smith, 1937 and a nomen oblitum. In so doing, Trimeresurus popeiorum Smith, 1937, and also becomes a nomen protectum and the binomen can be used for green pitvipers from Southeast Asia.

(n = 15). Nepal. NHMUK 1874.4.29.881 (adult female), “Himalayas”. — India. Meghalaya State: NHMUK 1872.4.17.137A (lectotype of Trimeresurus popeiorum, adult male), from Khasi Hillis; Sikkim State: NHMUK 1946.1.19.20 (holotype of Trimesurus elegans Gray, 1853, subadult female); West Bengal State: NHMUK 1872.4.17.377 (paralectotype of Trimeresurus popeiorum, adult male), NHMUK 1872.4.17.378 and NHMUK 1891.9.11.28–29 (three adult females) from Darjeeling District; Mizoram State: MZMU ZOO-GV 001 (adult male); Arunachal Pradesh: NCBS NRC-AA-4548 (adult male) from Kamlang WS., NCBS NRC-AA-4549 (adult male) and NCBS NRC-AA-4534 (adult female) from Eaglenest WS. China. Yunnan Province: CIB DL201070102 and CIB DL201070103 (paratypes of T. yingjiangensis, two adult males) from Heihe Village, Kachang Town, Yingjiang Country. — Myanmar. Kachin State: ZMMU NAP-09445 (adult male) from Inn Gyin Taung Mt., Indawgyi NR., Mohnyin Township; Sagaing Region: ZMMU NAP-09522 (subadult male) from Zalon Taung Mt., Ban Mau District.

Trimeresurus popeiorum differs from all other members of the subgenus Popeia by the combination of the following morphological characters: (1) dorsal surfaces in various shades of green, bluish-green or even turquoise blue; (2) in males, a vivid, wide bicolored ventrolateral stripe, bright and deep red below, white above; in females, stripe thin, white or yellow; (3) males with a conspicuous, bicolored postocular streak, thin and white ventrally, wide and bright red dorsally; streak absent in females; (4) eyes red to deep red in both males and females; (5) 21 dorsal scale rows at midbody, more or less strongly keeled in males, weakly keeled in females, scales of the first dorsal scale row always smooth; (6) first supralabial entirely separated from nasal scale by a distinct suture; (7) supraoculars much narrower than internasals; (8) internasals never in contact, separated by one or two scales; (9) 159–173 ventrals; 55–76 subcaudals, all paired; (10) hemipenes long and forked, reaching at least to the 25^th subcaudal, without spines; (11) 11–13 cephalic scales between the supraoculars in males, 10–12 in females; (12) relative tail length 0.18–0.21 in males, 0.14–0.19 in females.

(NHMUK 1946.1.19.20) of Trimesurus elegans Gray, 1853 (Fig. 4). A subadult female in a relatively good state of preservation after more than 150 years in preservative.

Body cylindrical, long, and laterally compressed (SVL 322 mm, TaL 62 mm, TL 384 mm, TaL/TL 0.161). Head triangular in dorsal view, elongate, clearly distinct from the neck (HL 18.4 mm, HL/SVL 0.06), snout elongate, flattened, and rounded when seen from above, rather rectangular when seen from lateral side, with a very distinct and sharp canthus rostralis; loreal pit present, triangular in shape. Eye average (ED 3.1 mm, SnL 5.2 mm, ED/SnL 0.58); pupil vertical, elliptic.

Dorsal scales in 25-21-15 rows; dorsal scales all moderately keeled, except the first row, the scales of which are smooth; 167 ventrals (plus single preventral); cloacal plate single; 61 subcaudals, all divided.

Rostral slightly visible from above, triangular; one large pair of enlarged internasals, separated by a one small scale; nostril completely included in nasal scale; nasal scale completely separated from the first supralabial; 1/2 small scales between nasal and second supralabial; scales on the upper snout surface and in the interorbital region smooth, irregular, barely imbricate; temporal and occipital scales strongly keeled; two elongate upper preoculars above the loreal pit; lower preocular forming the lower margin of the loreal pit; one supraocular on each side, large, broader than the internasals; 13 irregular cephalic scales between the supraoculars; one long, thin, crescent-like subocular scale (Fig. 4C); 9/9 supralabials, third largest and separated from the subocular by one scale on each side, fourth and fifth supralabials separated from subocular by one scale on each side; 2/2 postoculars (Fig. 4E, F); 13/12 infralabials, those of the first pair in contact with each other behind the mental, the first three pairs in contact with the single pair of chin shields. Five pairs of gulars aligned between the chin shields and the first preventral (Fig. 4D).

Body green, beneath paler, whitish; scales of the back moderate, smooth, not keeled, the lateral series rather broader, the first lateral series green, with a small white spot on the hind part of the upper edge forming an interrupted lateral line.

(NHMUK 1872.4.17.137A) of Trimeresurus popeiorum Smith, 1937 (Fig. 5). An adult male in a relatively good state of preservation after more than 150 years in preservative.

Body cylindrical, long, and laterally compressed (SVL 758 mm, TaL 167 mm, TL 925 mm, TaL/TL 0.181). Head triangular in dorsal view, elongate, clearly distinct from the neck, snout elongate, flattened, and rounded when seen from above (Fig. 5C), rather rectangular when seen from lateral side, with a very distinct and sharp canthus rostralis; loreal pit present, triangular in shape (Fig. 5E). Eye average; pupil vertically elliptic (Fig. 5E).

Dorsal scales in 23-21-15 rows; dorsal scales all moderately keeled, except the first row, the scales of which are smooth; 165 ventrals (plus single preventral); cloacal plate single; 70 subcaudals, all divided.

Rostral slightly visible from above, triangular; one large pair of enlarged internasals, separated by one small scale (Fig. 5C); nostril completely included in an entire nasal scale; nasal scale completely separated from the first supralabial; 1/2 small scales between nasal and the second supralabial; scales on the upper snout surface and in the interorbital region smooth, irregular, barely imbricate; temporal and occipital scales strongly keeled; two elongate upper preoculars above the loreal pit; lower preocular forming the lower margin of the loreal pit (Fig. 5E); one supraocular on each side, large, broader than the internasals; ten irregular cephalic scales on a line between the supraoculars (Fig. 5C); one long, thin, crescent-like subocular scale; 9/10 supralabials, third largest and separated from the subocular by one scale on each side; fourth and fifth supralabials separated from subocular by one or two scales on each side; 2/2 postoculars (Fig. 5E); 12/12 infralabials, those of the first pair in contact with each other behind the mental, the first three pairs in contact with the single pair of chin shields. Five pairs of gulars aligned between the chin shields and the first preventral (Fig. 5D).

Color uniform green above; paler green below. First row of scales on each side brown with yellow tip, second row yellow below the median keel. The narrow bicolor lateral stripe thus formed ending just behind head anteriorly and at vent posteriorly, incompletely developed on tail. Tip of tail pale reddish. Head without pattern or postocular stripe (Fig. 5A, B, F).

(CIB DL201070102) of Trimeresurus yingjiangensis Chen et al. (Fig. 6). Below we provide a brief redescription of the paratype of T. yingjiangensis (CIB DL201070102), examined by one of us (GV), in order to provide a detailed morphological data justifying the synonymy of this taxon with T. popeiorum, as proposed based on molecular data by Mirza et al. (2023).

An adult male in a very good state of preservation.

Body cylindrical, long, and laterally compressed; SVL 594 mm, TaL 148 mm, TL 742 mm, TaL/TL 0.199. Head triangular in dorsal view, elongate, clearly distinct from the neck (HL 29.8 mm, HW 18.1 mm, HL/SVL 0.05); snout elongate, flattened, and rounded when seen from above, rather rectangular when seen from lateral side, with a very distinct and sharp canthus rostralis; loreal pit present, triangular in shape. Eye average (ED 12.9 mm); pupil vertically elliptic.

Dorsal scales in 21-21-15 rows; dorsal scales all moderately keeled, except the first row the scales of which are smooth; 164 ventrals (plus single preventral); cloacal plate single; 76 subcaudals, all divided. Hemipenes long, reaching to 23–24 SC, forked opposite fifth to sixth subcaudal scale, no spines (Fig. 6H).

Rostral slightly visible from above, triangular, broader than high; one enlarged internasal on each side, internasals separated by one small scale behind the top of rostral (Fig. 6E); nostril completely included in the entire nasal scale, pentagonal, not divided, elongate, as long as high; nasal scale completely separated from the first supralabial (Fig. 6C, D); one internasal on each side, pentagonal, curved, wide, transversely elongate, separated by one small scale; two small scales between the nasal and the second supralabial; scales on the upper snout surface and in the interorbital region smooth, irregular, barely imbricate; 3/3 canthal scales, slightly larger than adjacent snout scales, bordering the canthus rostralis between the internasal and corresponding supraocular; temporal and occipital scales obtusely keeled; one relatively large triangular loreal between the upper preocular and the nasal; two elongate upper preoculars above loreal pit, lower one bordering the upper margin of loreal pit, upper one visible from above, both elongate and in contact with loreal; lower preocular forming lower margin of loreal pit; one supraocular on each side, long, much longer than wide, cephalic scales relatively small, irregular or slightly rounded, juxtaposed, flat and smooth; 11 irregular cephalic scales between the supraoculars; one long, thin, crescent-like subocular scale; occipital scales rhombohedral, distinctly but obtusely keeled; 2/2 small postoculars; 10/11 supralabials, first supralabial short, entirely separated from the nasal by a distinct suture; second supralabial tall, forming the anterior border of loreal pit; third supralabial largest and in contact with the subocular on each side; fourth and fifth supralabials much lower than the third one, separated from the subocular by one scale on each side (Fig. 6C, D); 12/12 infralabials, those of the first pair in contact with each other behind the mental, the first three pairs in contact with the single pair of chin shields. Five pairs of gulars aligned between the chin shields and the first preventral (Fig. 6F).

(Fig. 6 and Fig. 1D in Chen et al. 2019). The body is uniform deep bluish-green (bright grass-green in life), without darker areas or crossbands; a conspicuous, bicolored ventrolateral stripe extends from the beginning of the neck to the vent, pinkish-brown (bright deep red in life) ventrally, this color covering nearly the whole surface of scales of the first dorsal scale row except their upper posterior corner that is cream (white in life), same as the lower half of scales of the second row. The tail is bluish-green like the dorsum; the ventrolateral stripe extends up to the first third of the tail; near its end, the dorsal surface of the tail is irregularly mottled with pinkish-orange (rusty-red in life).

The dorsal surface of the head and temporal region are bluish-green like the body; the side of the head below the eye, i.e., the sides of the snout, nasal scale, anterior supralabials and lower temporals, is distinctly paler than the dorsal surface of the head, namely pale bluish-green (pale green in life); thin and short bicoloured postocular strip (red and white), runs from the posterior part of the subocular to the angle of the jaw. The chin and throat are pale sea-green (yellow green in life), uniform but with few faint darker areas on infralabials sometimes. The eye is grey but it was deep fire-red in life. The venter is uniform pale sea-green (yellow green in life); dorsal tail heavily mottled with dark red blotches, the dark red blotches contiguous posteriorly; the ventral surface of the tail is as the venter anteriorly, tip of tail red on dorsal while dark salmon on ventral.

This description is based on the ten specimens examined by us, supplemented by data from 15 specimens published by Das et al. (2016), Chen et al. (2019), Liu et al. (2022), and Mirza et al. (2023) (see Table S3).

The maximum known total length is 925 mm in males (NHMUK 1872.4.17.137), 848 mm in females (NHMUK 1891.9.11.28). The body is robust but relatively slender in males, thicker in large females, laterally compressed; head triangular, elongate, wide posteriorly, flattened in males, moderately thick in females, clearly distinct from the neck; snout elongate, distinctly flattened, rounded seen from above, angular and obliquely truncated in profile view, with a distinct canthus rostralis; nostril piercing in the middle of nasal scale; eye average, amounting for 0.9–1.2 times in males and 0.7–1.1 times in females the distance between the lower margin of eye and upper lip border; tail average to long, progressively tapering and distinctly prehensile; ratio TaL/TL 0.140–0.219, with a weak sexual dimorphism (males: 0.179–0.219; females: 0.140–0.191).

Dorsal scales rhombohedral, more or less strongly keeled in males, weakly keeled in females, in 21-21-15 (73%) or 23-21-15 (23%) rows (exceptionally in 25-21-15 rows; 4%); scales of the first dorsal scale row smooth and not enlarged; 159–173 ventrals (plus one or two preventrals), rounded; 57–76 paired subcaudal scales with overlapping sexual dimorphism (64–76 in males, 57–70 in females); cloacal plate entire.

Rostral much broader than high, triangular, well visible from above; nasal subrectangular, entire, elongate, longer than high; one internasal on each side, pentagonal, curved, wide, transversely elongate, separated by one (90%) or rarely two (10%) small scales; four or five canthal scales, slightly larger than adjacent snout scales, bordering the canthus rostralis between the internasal and corresponding supraocular; one relatively large triangular loreal between the upper preocular and nasal; two upper preoculars above loreal pit, lower one bordering the upper margin of loreal pit, upper one visible from above, both elongate and in contact with loreal; lower preocular forming lower margin of loreal pit; one supraocular on each side, entire, elongate and rather narrow, about 2.7–3.3 times longer than wide, about 0.5–0.9 times as wide as the internasals, indented on their inner margins by the upper head scales; cephalic scales relatively small, irregular or slightly rounded, juxtaposed, flat and smooth; 10–13 cephalic scales on a line between supraoculars; occipital scales rhombohedral, moderately or more frequently distinctly keeled in males, smooth or weakly keeled in females; temporals rhombohedral, distinctly keeled or less frequently smooth in males, always smooth in females; on each side, one thin, elongate, subocular scale, crescent-shaped; 2–3 small postoculars; 9–13 (usually 10–12) supralabials; first supralabial short, entirely separated from the nasal by a distinct suture; second supralabial tall, always forming the anterior border of loreal pit, separated from the nasal by one or two small scales; third supralabial the longest and highest, rather tall, usually separated from the subocular by one scale on each side (77.8%), rarely in contact (22.2%); fourth supralabial as long as high, lower than third supralabial, separated from subocular by one scale; fifth supralabial smaller than the fourth one, separated from the subocular by one or two scales of similar size; 10–15 (generally 12 or 13) infralabials, those of the first pair in contact with each other, the first three pairs in contact with the chin shields.

In life, the body is uniform bright green, grass-green, deep green, bluish-green or even turquoise blue (Wall 1909). In preservative, the general background color remains green or turns to bluish-green, reddish-brown, brown or even black. Generally, numerous faint, dark transversal bands due to the dark brown or dark grey skin between the dorsal scales; in males, a broad and conspicuous, bicolored ventrolateral stripe, bright red, deep red or rusty brown on its lower part covering the lower half of the scales of the first dorsal scale row, white or whitish-yellow above on the upper posterior half of scales of the first row and on the lower part of scales of the second row, extends from the angle of the mouth to the base of the tail; in females, the ventrolateral stripe is thinner, white or creamish-yellow anteriorly, white or cream posteriorly, conspicuous and extends from the neck to the base of the tail on the first dorsal scale row. The tail is of the same green color than the dorsum, irregularly mottled with reddish-brown or rusty brown, without a clear demarcation between the red and green colors, entirely reddish-brown posteriorly; the ventrolateral stripe extends up to the first third to half-length of the tail.

The dorsal surface of the head and temporal region are uniform green or bluish-green like the body; the side of the head below the eye, i.e., the lower sides of the snout, nasal scale, and anterior supralabials and lower temporals, is distinctly paler than the dorsal surface of the head, generally pale green, yellowish-green or pale bluish-green; in males, a vivid, broad, bicolored postocular streak, the lower part narrow and white, the upper part broad and bright red, rusty-red or brownish-red, present at any age, extends from the postoculars obliquely towards the angle of the mouth then to the lower side of the neck where it connects to the ventrolateral stripe; in females, the postocular streak is usually absent, or present as a white and thin line. The chin and throat are generally pale yellowish-green, with infralabials sometimes bright yellow or marked with green. The eye is bright red, fire-red or deep red in life in adult specimens of both sexes (Fig. 7A–E).

The venter is uniform yellowish-green or pale green; tips of ventrals red as the lower part of the ventrolateral stripe. The ventral surface of the tail is as the venter anteriorly, becoming rusty-red on its posterior half to third.

The organ is long and thin, deeply forked, extending to the 25^th or 26^th subcaudal scale in situ, forked opposite at the level of the sixth to eighth subcaudal scales; the base of the organ, up to the point of bifurcation, is entirely smooth with longitudinal folds, except for the sulcus spermaticus; from the point of bifurcation up to the tip of the organ, each fork is finely calyculate. The sulcus is prominent; it divides near the base of the organ and ends near the tip of the fork (based on Smith 1937; Chen et al. 2019; Mirza et al. 2023; our data).

Maxilla with one functional and 5–6 replacement fangs; palatine with four teeth, pterygoid with eight teeth, 10–12 dentary teeth (based on Mirza et al. 2023; and our data).

(Fig. 1). Based on our definition of T. popeiorum, we establish its range as follows: India (northeasternern part of the country, in the states of Sikkim, West Bengal, Meghalaya, Assam, Arunachal Pradesh, and Mizoram); Bhutan (south); Nepal (east); Bangladesh (southeast: Chittagong Province), China (western Yunnan Province: Dehong Dai and Jingpo Autonomous Prefecture), and Myanmar (northern and southwestern parts of the country, in Kachin and Chin States, and Sagaing Region). The southernmost record for this species is from Ngape Township, Rakhine Yoma, Rakhine State, Myanmar (our data). The occurrence of this species in the states of Nagaland and Manipur, India, is expected.

Trimeresurus popeiorum inhabits hilly and montane regions covered with humid tropical and subtropical submontane and montane evergreen forests and semi-evergreen forests. Chen et al. (2019) stated that at the type locality of T. yingjiangensis in Yunnan Province this species prefers to inhabit sites near streams at an elevation of about 1000 m, and can be found perched on branches waiting for prey in ambush. In contrast, in Myanmar, Liu et al. (2022) found individuals perched on small or big trees at elevations of only 155–176 m, and no rivers or streams were nearby. In the Dampa Tiger Reserve, Mizoram State, India, Malsawmdawngliana et al. (2022) noted that this species was quite common in their study area. These authors encountered more than ten individuals among roadside vegetation and along forest trails, and a female perched in ambush position on a small guava tree near the guest house. In Mizoram State, one of the authors of the present paper (GV) found a large female, on a tree trunk at about 1.7 m above the ground, in a secondary forest at an elevation of about 800 m. Moreover, T. popeiorum appears to be common across lower-elevation forests of Arunachal Pradesh State. Most individuals were observed perched on vegetation along roads, but we observed a higher number of individuals along streams. The species is found in sympatry with T. salazar Mirza, Bhosale, Phansalkar, Sawant, Gowande & Patel in Arunachal Pradesh and Assam states and T. erythrurus (Cantor) in Meghalaya and Mizoram states.

AUP-00180, an adult male from Siriphum Waterfall, Doi Inthanon National Park (18.5467°N, 98.5124°E; elevation 1450 m), Chiang Mai Province, Thailand; collected by P. Pawangkhanant, C. Suwannapoom, and N. A. Poyarkov on 30 August 2017.

(n = 8).

Chiang Mai Province: AUP-00178 (adult male), AUP-01574 (adult female) data same with holotype; FMNH 178656 (adult male), collected by O.G. Young, and NHMUK 1937.2.1.25 (adult male), collected by M.A. Smith in 1937, no specific locality. Lampang Province: IRSNB 2736 (formerly IRSNB 16545; adult male) and IRSNB 2737 (formerly IRSNB 16546; subadult female) from Chae Son NP. Laos. Phongsaly Province: FMNH 14430 (adult male) from Ban Muangyo, collected by R.E. Wheeler on 9 May 1929; MNHN-RA-2004.0262 (adult male), between Nathen and Long Nai, collected on 26 July 2004.

(n = 21). Thailand. Chiang Mai Province: MNHN-RA-1987.3836 (adult male), PSGV S0062 (subadult male), and AUP-00181, PSGV S0063 (two subadult females) from Doi Inthanon NP.; USNM 84757 (subadult female) from Khun Chae NP.; NHMW 27947 (adult male), FMNH 178655, FMNH 178658 (two subadult males), and NHMUK 1937.2.1.24, NHMUK 1937.2.1.26 (two subadult females), no specific localities. — Laos. Luangphrabang or Xaignabouli provinces: NHMUK 1862.7.28.4 (adult male) and NHMUK 1862.7.28.1 (adult female), “Lao Mountains, Cochinchina”, now Luangphrabang Range, western Laos, probably between Paklay (Xaignabouli Province) and Luangphrabang (Luangphrabang Province). — Myanmar. Mon Sate: CAS 222195, CAS 240640 (two adult males), and CAS 216609 (subadult male) all from Kyaik Hti Yo WS. Bago Reigon: CAS 205847 (adult female) from Bago Yoma City. Kayah State: NHMB 2596-97, NHMW 23923:1-2, and ZMB 11637 (five adult males) from Monts Karen Mt.

The new species name “lanna” represents a noun in apposition and is given in reference to the Lan Na Kingdom, or “Kingdom of a Million Rice Fields”. The Kingdom of Lan Na, also known as Lannathai, was centered in present-day northern Thailand from the 13^th–18^th centuries. The territories and cultural influence of the Lan Na Kingdom spread from easternmost Myanmar to northern Laos and southernmost present-day Yunnan of China, a geographic area that matches well the range of the new species. Though eventually the Lan Na Kingdom was united with the Siamese State in the early 19^th century, its culture had a profound influence on different parts of northern Indochina. We suggest the following common names for the new species: “Ngu Khiew Hang Mai Thong Khiew Nua” (เขียวหางไหม้ท้องเขียวเหนือ) (in Thai), “Lán nà zhú yè qīng” (蘭納竹叶青) (in Chinese), “Lanna green pitviper” (in English), “Trimérésure vert du Lanna” (in French), “Lanna Bambusotter” (in German), and “Chiangmaiskaya bambukovaya kufiya” (in Russian).

Trimeresurus lanna differs from other members of the subgenus Popeia by the combination of the following morphological characters: (1) dorsal surfaces deep green, without crossbands; (2) in males, a vivid, wide, bicolored ventrolateral stripe, bright and deep red below, white above; in females, ventrolateral stripe thin, pale yellow anteriorly, whitish posteriorly; (3) in males, a conspicuous, bicolored postocular streak, white and thin ventrally, broad and bright red dorsally, covering two or three temporal scales; in females, streak absent or only white; (4) eyes red to deep red in both males and females; (5) 21 (93.3%) or rarely 20 (6.7%) dorsal scales rows at midbody, strongly keeled except those of the first dorsal scale row, always smooth; (6) 145–167 ventral plates (145–167 in males; 157–166 in females); 56–75 paired subcaudal scales with weak sexual dimorphism (59–75 in males, 56–64 in females); (7) first supralabial entirely separated from the nasal scale by a distinct suture; (8) supraoculars relatively narrow, narrower than internasals, separated by 10–13 cephalic scales; (9) internasals never in contact, separated by one or two scales; (10) hemipenes long and forked, reaching at least 25^thSC, without spines; (11) 9–14 cephalic scales between the supraoculars in males, 11–13 in females; (12) relative tail length 0.18–0.21 in males, 0.16–0.17 in females.

(Fig. 8A–D). An adult male, specimen in a very good state of preservation.

Body cylindrical, long, and laterally compressed; SVL 640 mm, TaL 175 mm, TL 815 mm, TaL/TL 0.215. Head triangular in dorsal view, elongate, clearly distinct from the neck (HL 36.8 mm, HL/SVL 0.06); snout elongate, flattened, and rounded when seen from above, rather rectangular when seen from lateral side, with a very distinct and sharp canthus rostralis; loreal pit present, triangular in shape. Eye average (ED 4.5 mm, SnL 11.3 mm, ED/SnL 0.40); pupil vertically elliptic.

Dorsal scales in 23-21-15 rows; dorsal scales all moderately keeled, except the first row the scales of which are smooth; 157 ventrals (plus single preventral); cloacal plate single; 71 subcaudals, all divided.

Rostral slightly visible from above, triangular, broader than high; one enlarged internasal on each side, internasals separated by one small scale behind the top of rostral (Fig. 8A); nostril completely included in the entire nasal scale, pentagonal, undivided, elongate, as long as high; nasal scale completely separated from the first supralabial (Fig. 8C); one internasal on each side, pentagonal, curved, wide, transversely elongate, separated by one small scale; two small scales between the nasal and the second supralabial; scales on the upper snout surface and in the interorbital region smooth, irregular, barely imbricate; 3/3 canthal scales, slightly larger than adjacent snout scales, bordering the canthus rostralis between the internasal and corresponding supraocular; temporal and occipital scales obtusely keeled; one relatively large triangular loreal between the upper preocular and the nasal; two elongate upper preoculars above loreal pit, lower one bordering the upper margin of loreal pit, upper one visible from above, both elongate and in contact with loreal; lower preocular forming lower margin of loreal pit; one supraocular on each side, long, much longer than wide, 0.7/0.8 times as wide as the internasals; cephalic scales relatively small, irregular or slightly rounded, juxtaposed, flat and smooth; 11 irregular cephalic scales between the supraoculars; one long, thin, crescent-like subocular scale; occipital scales rhombohedral, distinctly but obtusely keeled; 2/2 small postoculars; 10/10 supralabials, first supralabial short, entirely separated from the nasal by a distinct suture; second supralabial tall, forming the anterior border of loreal pit; third supralabial the largest and in contact with the subocular on each side; fourth and fifth supralabials much lower than the third one, separated from the subocular by one scale on each side (Fig. 8C); 12/12 infralabials, those of the first pair in contact with each other behind the mental, the first three pairs in contact with the single pair of chin shields. Five pairs of gulars aligned between the chin shields and the first preventral (Fig. 8B).

(Fig. 8A–D). The body is uniform deep bluish-green (bright grass-green in life), without darker areas or crossbands; a conspicuous, bicolored ventrolateral stripe extends from the beginning of the neck to the vent, pinkish-brown (bright deep red in life) ventrally, this color covering nearly the whole scales of the first dorsal scale row except their upper posterior corner that is cream, and dorsally cream (white in life) on the lower half of scales of the second row. The tail is bluish-green like the dorsum; the ventrolateral stripe extends up to the first third of the tail; near its end, the dorsal surface of the tail is irregularly mottled with pinkish-orange (rusty-red in life).

The dorsal surface of the head and the temporal region are bluish-green like the body; the side of the head below the eye (i.e., the sides of the snout, nasal scale, anterior supralabials and lower temporals) is distinctly paler than the dorsal surface of the head, namely pale bluish-green (pale green in life); a broad, bicolored postocular streak, its ventral part narrow and cream (white in life) covering the lower row of temporals, its upper part, broader (two rows of temporals), pinkish-brown (bright deep red in life), extends from the postoculars obliquely towards the angle of the mouth but the postocular streak does not connect with the ventrolateral stripe. The chin and throat are pale sea-green (pale green in life), uniform but with few faint darker areas on infralabials sometimes. The eye is grey but it was deep fire-red in life. The venter is uniform pale sea-green (pale green in life); tips of ventrals of the same green color, not red. The ventral surface of the tail is as the venter anteriorly, becoming pinking-orange in its posterior quarter (reddish-brown in life).

(based on adult male CAS 240640, Fig. 8H). The organ is long and thin, deeply forked, extending to the 18^th subcaudal scale in situ, forked at the level of fifth to sixth subcaudal scales; the base of the organ, up to the point of bifurcation, is entirely smooth with longitudinal folds, except for the sulcus spermaticus; from the point of bifurcation up to the tip of the organ, each fork is finely calyculate. The sulcus is prominent; it divides near the base of the organ and ends near the tip of the fork.

The longest known specimen is 845 mm long (SVL 709 mm, TaL 136 mm; female, NHMUK 1862.7.28.1). The longest known male is 815 mm long (SVL 640 mm, TaL 175 mm; holotype). The body is robust, relatively slender in males, thicker in large females, laterally compressed; head triangular, elongate, wide posteriorly, flattened in males, moderately thick in females, clearly distinct from the neck; snout elongate, distinctly flattened, rounded seen from above, angular and obliquely truncated in profile view, with a distinct canthus rostralis; nostril piercing in the middle of nasal scale; eye average, amounting for 0.9–1.2 times in males and 0.7–1.0 times in females the distance between the lower margin of eye and upper lip border; tail average to long, progressively tapering and distinctly prehensile; ratio TaL/TL 0.149–0.211, with a sexual dimorphism (males: 0.182–0.211; females: 0.149–0.173).

Dorsal scales in 23-21-15 (50%), 21-21-15 (36.7%), or rarely 25-21-15 (10%) and 22-21-15 (3.3%) rows, moderately or strongly keeled; scales of the first dorsal scale row smooth and not enlarged; 145–167 ventral plates (plus one or two preventrals), rounded; 56–75 subcaudal scales with a weak sexual dimorphism (59–75 in males, 56–64 in females), all paired; total number of VEN+SC: 213–241, without sexual dimorphism; cloacal plate entire.

The head scalation is as described for the holotype, with the following variation: internasals separated by one (84%) or rarely two (16%) small scales; three or four canthal scales bordering the canthus rostralis between the internasal and corresponding supraocular; one supraocular on each side, entire and rather narrow, about 0.7–0.9 times as wide as the internasals, indented on their inner margins by the upper head scales; cephalic scales juxtaposed, flat and smooth; 10–13 cephalic scales on a line between supraoculars; occipital scales rhombohedral, distinctly obtusely keeled in males, weakly keeled or even smooth in females; temporals generally moderately but distinctly keeled (80%), rarely smooth (20%) in males, always smooth in females; on each side, one thin, elongate subocular scale, crescent-shaped; two or three small postoculars; 9–11 supralabials; third supralabial the longest and highest, rather tall, in contact with the subocular or separated from this latter scale by one scale; fourth supralabial separated from the subocular by one scale in all examined specimens; fifth supralabial smaller than the fourth one, separated from the subocular by one or two scales of similar size; 10–14 (generally 12–13) infralabials, those of the first pair in contact with each other.

(Figs 8, 9). In life or in freshly preserved animals, the body is uniform bright green, grass-green, emerald-green or deep green (in preservative, the general background color remains green or turns to bluish-green); no dark transversal crossbands or white vertebral dots; in males, the broad, bicolored ventrolateral stripe is as described above, namely bright red, deep red or rusty brown on its lower part, white or whitish-yellow above, always present and conspicuous; in females, the ventrolateral stripe is absent or thin and faint, white or cream. The tail is of the same green color than the dorsum, irregularly mottled with reddish-brown or rusty brown posteriorly.

The dorsal surface of the head and the temporal region are uniform green or bluish-green like the body; the sides of the head below the eye, i.e., the lower sides of the snout, nasal scale, and anterior supralabials and lower temporals are distinctly paler than the dorsal surface of the head, pale green or pale bluish-green; in males, the vivid, broad, bicolored postocular streak, white ventrally, bright red, rusty-red or brownish-red dorsally, is always present; in females, the postocular streak is usually absent (85.7%) or present as a white and thin line (14.3%). The chin and throat are pale green or pale sea-green, uniform or with infralabials marbled with green. The eye is bright red, fire-red or deep red in specimens of both sexes. The venter is uniform pale green or pale sea-green; tips of ventrals green. The ventral surface of the tail is as the venter anteriorly, becoming rusty-red on its posterior half to third.

We here compare T. lanna with the four other species of the subgenus Popeia (T. nebularis, T. phuketensis, T. popeiorum, and the T. sabahi complex). The main diagnostic characters separating the new species from these four species are summarized in Table S4.

Trimeresurus lanna is morphologically very similar to T. popeiorum but it is distinguished from this latter species by having: (1) lower max TL in males (815 mm vs. 925 mm), but higher max TL in females (884 mm vs. 854 mm); (2) slightly lower number of ventral plates in males (145–167, x̄ = 159.9 vs. 162–171, x̄ = 165.7; p = 0.0004); (3) slightly lower total number of VEN+SC in males (213–241, x̄ = 227.81 vs. 229–240, x̄ = 234.71; p = 0.0004); (4) bicolor postocular streak in males broad, covering 2–3 temporal scales vs. narrow, covering 1–2 temporal scales; (5) temporals strongly keeled in males (vs. feebly keeled).

Trimeresurus lanna is distinguished from T. nebularis by having: (1) eye color deep red in both sexes vs. usually green; (2) bicolor postocular streak present in males vs. absent; presence of a white ventrolateral stripe in females vs. absent; (3) lower max TL in both sexes (815 mm in males, 854 mm in females vs. 1002 mm in males, 948 mm in females); (4) higher total number of VEN+SC in both sexes (213–241, x̄ = 228.1 vs. 210–218, x̄ = 214.0 in males; p = 0.007; 213–229, x̄ = 221.2 vs. 197–210, x̄ = 205.6 in females; p = 0.021).

Trimeresurus lanna differs from T. phuketensis by having: (1) higher max TL in both sexes (815 mm in males, 854 mm in females vs. 640 in males,748 mm in females); (2) lower total number of VEN+SC in both sexes (213–241, x̄ = 227.8 vs. 242–249, x̄ = 246.4 in males; p = 0.0032; 213–229, x̄ = 221.2 vs. 226–237, x̄ = 230.6 in females; p = 0.015); (3) eye color deep red in both sexes vs. copper; (4) body without dorsal crossbands vs. present and conspicuous.

Trimeresurus lanna can be further differentiated from the five subspecies of T. sabahi as follows:

– from T. s. barati by having: (1) 21 dorsal scales rows at midbody (vs. 17–19); (2) eye color deep red in both sexes vs. deep orange; (3) bicolor postocular streak present in males vs. absent; (4) slightly higher max TL in both sexes (815 mm in males, 854 mm in females vs. 740 mm in males, 720 mm in females);

– from T. s. buniana by having: (1) dorsum uniform bright green vs. dark bluish-green or verdigris with conspicuous reddish-brown or violet, irregular crossbands; (2) eye color deep red in both sexes vs. copper; (3) bicolor postocular streak (red plus white) present in males vs. postocular streak reddish-brown; (4) slightly lower ratio TaL/SVL in both sexes (0.18–0.21, x̄ = 0.20 vs. 0.22–0.23, x̄ = 0.22 in males; p = 0.007; 0.14–0.17, x̄ = 0.16 vs. 0.21 in females); (5) lower total number of VEN+SC in both sexes (213–241, x̄ = 228.5 vs. 246–250, x̄ = 248.0 in males; p = 0.0004; 213–229, x̄ = 221.4 vs. 231 in females);

– from T. s. fucatus by having: (1) slightly lower ratio TaL/TL in both sexes (0.18–0.21, x̄ = 0.20, vs. 0.19–0.24, x̄ = 0.22 in males; p = 0.000002; 0.14–0.17, x̄ = 0.16 vs. 0.16–0.19, x̄ = 0.17 in females; p = 0.05; (2) eye color deep red in both sexes vs. copper; (3) dorsal crossbands absent in males vs. present; (4) white vertebral spots absent vs. present; (5) bicolor postocular streak wide (red plus white) present in male vs. sometimes absent, or white, or white with its upper part dark red and thin;

– from T. s. toba by having: (1) eye color deep red in both sexes vs. deep orange; (2) ventrolateral stripe bicolor, wide in males vs. white and thin; (3) slightly higher max TL in both sexes (815 mm in males, 854 mm in females vs. 730 mm in males, 798 mm in females);

– from T. s. sabahi by having: (1) bicolor postocular streak present in males vs. absent; (2) slightly lower TaL/TL ratio in both sexes (0.18–0.21, x̄ = 0.20 vs. 0.19–0.24, x̄ = 0.21 in males; p = 0.05; 0.14–0.17, x̄ = 0.16 vs. 0.17–0.18, x̄ = 0.18 in females; p = 0.04); (3) slightly higher total number of VEN+SC in females (213–229, x̄ = 221.2 vs. 212–217, x̄ = 214.7; p = 0.034).

Furthermore, in western Laos and northern Thailand T. lanna may be recorded in sympatry with the superficially similar T. (Viridovipera) gumprechti David et al., 2002. The two species can be easily distinguished from each other by hemipenial morphology: in the new species hemipenes are long, slender and deeply forked, reaching in situ at least the 25^th subcaudal, lacking spines (vs. hemipenes short, thick, barely forked and strongly spinous, extending at most up to the 13^th–15^th subcaudals in T. gumprechti). Furthermore the eye of T. lanna is always deep red or fire-red in life in both sexes (vs. eye in life red in males, copper or yellow in females in T. gumprechti); and the head is flat with a distinctly obliquely truncated snout (vs. snout barely obliquely truncated, rather rectangular or rounded in T. gumprechti).

Trimeresurus lanna is currently known from a large expanse in northern Indochina around the infamous “Golden Triangle”, including southeastern Myanmar. Based on our data, we establish the distribution of this species as follows: Myanmar (southeastern part of the country: Mon and Kayah States, and the Bago Region). China (Yunnan Province: Puer, Jinghong, and Mengla Counties). Laos (Houaphan, Luangnamtha, Luangphrabang, Oudomxay, Phongsali, Xaignabouli, and Vientiane Provinces). Thailand (northern and western parts of the country: Chiang Mai, Chiang Rai, Kamphaeng Phet, Kanchanaburi, Lampang, Mae Hong Son, Nan, Tak, and Uthai Thani Provinces; the southern limit of the range is in Sri Sa Wat District, Kanchanaburi). The occurrence of the new species in Shan and Kayin States of Myanmar, in the province of Bokeo in Laos, and in Phayao and Phrae Provinces in Thailand, is strongly anticipated.

Trimeresurus lanna typically inhabits humid tropical submontane primary evergreen and secondary forests, subtropical montane evergreen, semi-evergreen and mixed forests, as well as mixed secondary submontane forest, generally at elevations between 600 and 2000 m but it can be found at much lower elevations in suitable cool and humid habitats. It feels also at home in moist monsoon and other deciduous forests, bamboo thickets, and plantations. In Chiang Mai Province, Thailand, it is normally found in evergreen submontane and montane forest at elevations between 500 and 1700 m. It occurs in regions where winter temperatures can be as low as 3–7°C. The preferred microhabitats of this pitviper are markedly cool, humid, and shaded places. It is generally associated with dense shrubs, thickets, bushes, scrubs, the foliage of low trees, and tall grasses. This species is generally found in the low, humid vegetation of riparian areas, especially in the vicinity of freshwater habitats. Wogan and Chan-ard (2012, 2022; reported as T. popeiorum) stated that individuals can often be found hanging over streams and in bushes. It is also often seen along the edges of forest clearings and sides of forest tracks.

Trimeresurus lanna is arboreal, crepuscular, and nocturnal. It is usually found coiled up above small streams or sometimes as high as 4 or 5 m above the ground. In Tak Province, Thailand, GV observed an individual on the ground in a submontane forest. In Oudomxay Province, Laos, Nguyen et al. (2020; reported as T. popeiorum) found individuals perched at night (1900–2300 h) from 0.5–4 m above the ground along a road in a forested valley. Still in Laos, Maury et al. (2022) recorded many specimens during the day and at night between elevations of 450–1300 m, the species being more abundant near streams and other freshwater habitats. Snakes were perched high in trees, between about 2 m and 10 m during dry days, where they were probably resting. At night, especially during rainy weather, specimens were found much closer to the ground, at heights between 10 cm and 2 m. This pitviper feeds mainly on frogs and lizards, especially geckos, also small mammals, and birds. It is ovoviviparous. On Doi Suthep Mt., Chiang Mai Province, Thailand, one of the authors (P. Pawangkhanant) observed a mating pair in October; mating started after a big rainfall around 1750 h, the adult male stayed near the female for almost two weeks. Newborns have been observed along a small stream on Doi Inthanon NP, Chiang Mai Province, Thailand (elevation ca. 1230 m) in late April to May.

Further research is required to clarify the extent of the distribution, population size and trends before the conservation status of this new species can be assessed. Trimeresurus lanna is distributed over a large area including many protected areas. Across its range the new species generally seems to be quite common. The density of some populations may be high as Maury et al. (2022) found up to 25 individuals in a single evening. These authors stated that it was common to find over 15 individuals at the same place in secondary forest. Thus, we tentatively suggest T. lanna be considered a species of Least Concern (LC) following the IUCN’s Red List categories (IUCN Standards and Petitions Committee 2019).

ZMMU Re-17668 (adult male) from Suan Phueng District, Ratchaburi Province, Thailand (13.55358°N, 99.20528°E; elevation 640 m) collected by P. Pawangkhanant on 7 June 2019.

(n = 4). Thailand. ZMMU Re-17671 (adult female) collected on 3 June 2019, same information than the holotype. — Myanmar. Tanintharyi Region: NHMUK 1924.5.20.38 (adult male) from Paya Taung Mt., Dawei City; CAS 247870 (adult male) and CAS 247754 (adult female) from Kawthaung District.

(n = 5). Thailand. ZMMU Re-17672 (subadult male) and ZMMU Re-17670 (subadult female) collected on 3 June 2019, ZMMU Re-17673 (subadult male) collected on 17 July 2019, all same information with holotype. — Myanmar. Taninthayi Region: NHMUK 1856.5.6.105 from Myeik Is., Myeik District; NHMUK 1940.3.9.43 (adult male) from Kisseraing Is, Myeik District.

(n = 4). Myanmar. Tanintharyi Region: USNM 587920 (subadult female), USNM 587921 (adult male) from Kawthaung District; USNM 587588 (adult female) Lenya Area; and USNM 587919 (adult female) from Ywahilu Village (see detail from Mulcahy et al. 2017).

The species name “tenasserimensis” is a modern Latin toponymical adjective in nominative singular, adopting the masculine gender of the genus name Trimeresurus, combining the name of the Tenasserim Mountain Range in western Thailand and southeastern Myanmar, where the new species occurs, and the Latin suffix -ensis (-is, -e), meaning “from”. The species nomen therefore means “from Tenasserim”. We suggest the following common names for the new species: “Ngu Khiew Hang Mai Thong Khiew Tanao Sri” (งูเขียวหางไหม้ท้องเขียวตะนาวศรี) (in Thai), “Dān nà shā lín zhú yè qīng” (丹那沙林竹叶青) (in Chinese), “Tenasserim green pitviper” (in English), “Tenasserim Bambusotter” (in German), “Trimérésure vert du Tenasserim” (In French) and “Tenasserimskaya bambukovaya kufiya” (in Russian).

Trimeresurus tenasserimensis differs from other members of the subgenus Popeia by the combination of the following morphological characters: (1) dorsal surfaces deep green, with faint dark, interstitial crossbands; (2) in males, a wide, bicolored ventrolateral stripe, bright red ventrally, white dorsally; in females, ventrolateral stripe thin, pale yellow anteriorly, whitish posteriorly; (3) in males, a conspicuous, bicolored postocular streak, white and thin ventrally, broad and bright red dorsally, covering two or three temporal scales; in females, streak absent or only white; (4) eyes red to deep red in both males and females; (5) 21 dorsal scales rows at midbody, strongly keeled except those of the first dorsal scale row, always smooth; (6) 159–176 ventrals (159–170 in males, 154–176 in females); 57–74 subcaudals with slightly overlapping sexual dimorphism (66–74 in males, 57–66 in females), all paired; (7) first supralabial entirely separated from the nasal scale by a distinct suture; (8) supraoculars relatively narrow, narrower than internasals, separated by 9–11 cephalic scales; (9) internasals not in contact, separated by one scale; (10) 10–11 cephalic scales between the supraoculars in both sexes; (11) relative tail length 0.20–0.23 in males, 0.14–0.16 in females.

(Fig. 10A–F). Adult male, specimen in a good state of preservation. Especially, it had still retained its color in life at the time of writing this paper.

Body cylindrical, long, and laterally compressed (SVL 502 mm, TaL 127 mm, TL 629 mm, TaL/TL 0.202). Head triangular in dorsal view, elongate, clearly distinct from the neck (HL 30.2 mm, HL/SVL 0.06), snout elongate, flattened and rounded when seen from above, rather rectangular when seen from lateral side, with a very distinct and sharp canthus rostralis; loreal pit present, triangular in shape. Eye average (ED 3.3 mm, SnL 8.4 mm, ED/SnL 0.39); pupil vertically elliptic.

Dorsal scales in 21-21-15 rows; dorsal scales all moderately keeled, except the first row of which scales are smooth; 160 ventrals (plus two preventrals); cloacal plate single; 66 subcaudals, all divided.

Rostral slightly visible from above, triangular; one internasal on each side, pentagonal, distinctly transversely elongate, internasals separated by one small scale behind the top of rostral (Fig. 10E); nostril completely included in nasal scale, pentagonal, entire, elongate, as long as high; nasal scale completely separated from the first supralabial; one small scale between nasal and the second supralabial; scales on the upper snout surface and in the interorbital region smooth, irregular, barely imbricate; 4/4 canthal scales, slightly larger than adjacent snout scales, bordering the canthus rostralis between the internasal and corresponding supraocular; temporal and occipital scales distinctly but obtusely keeled; one relatively large triangular loreal between the upper preocular and the nasal; two elongate upper preoculars above loreal pit, lower one bordering the upper margin of loreal pit, upper one visible from above, both elongate and in contact with loreal; lower preocular forming lower margin of loreal pit (Fig. 10C, D); one supraocular on each side, long, much longer than wide, 0.7/0.8 times as wide as the internasals; cephalic scales relatively small, irregular or slightly rounded, juxtaposed, flat and smooth; ten irregular cephalic scales between the supraoculars; one long, thin, crescent-like subocular scale; occipital scales rhombohedral, distinctly but obtusely keeled (Fig. 10E); 2/2 small postoculars; 10/11 supralabials, first supralabial short, entirely separated from the nasal by a distinct suture; second supralabial tall, forming the anterior border of loreal pit; third supralabial largest and in contact with the subocular on each side; fourth and fifth supralabials, much lower than the third one, separated from the subocular by one scale on each side (Fig. 10C, D); 13/12 infralabials, those of the first pair in contact with each other behind the mental, the first three pairs in contact with the single pair of chin shields. Five pairs of gulars aligned between the chin shields and the first preventral (Fig. 10F).

(Fig. 10A–F). The body is uniform bright grass-green, with faint darker areas forming crossbands due to the darker interstitial skin; a conspicuous, bicolored ventrolateral stripe extends from the beginning of the neck to the vent, bright deep red ventrally, this color covering a large part of the scales of the first dorsal scale row except their upper posterior corner or half that is white, and dorsally white on the lower half of scales of the second row; the large white areas on the red first dorsal scale row makes this ventrolateral stripe seeming red alternately with white; elongate white dots, faint, one scale long, widely spaced on scales of the vertebral row, more visible anteriorly. The tail is bright green like the dorsum, ornate above and on its sides by about 15, distinct large blotches crossing the vertebral line of the tail, hexagonal or subrectangular, bright rusty-red, extending downwards to the mid-height of the sides, in contact each with the other on the vertebral row, progressively fused posteriorly as an irregular stripe; the ventrolateral stripe of the body extends at least up to half of the tail length before vanishing as red blotches; near its end, the dorsal surface of the tail is irregularly mottled with rusty-red.

The dorsal surface of the head and the temporal region are bright grass-green like the body; the side of the head below the eye, i.e., the sides of the snout, nasal scale, anterior supralabials and lower temporals, is paler green than the dorsal surface of the head; a broad, bicolored postocular streak, its ventral part narrow and white, covering the lower part of the second row and, posteriorly, the first lower row of temporals, its upper part, broader, covering the second and third rows of temporals, bright rusty-red, extends from the postoculars obliquely towards the angle of the mouth; the postocular streak does not connect with the ventrolateral stripe. The chin and throat are uniform pale green. The eye is deep brownish-red.

The venter is uniform pale green; tips of ventrals of the same green color, not red. The ventral surface of the tail is as the venter anteriorly, with the bright red ventrolateral stripe on each side, becoming greenish-grey near its tip with reddish-brown dots but not completely reddish-brown.

(see also Table S3). The longest-known specimen is 736 mm long (SVL 587 mm, TaL 149 mm; male, CAS 247870). The longest-known female is 532 mm long (SVL 452 mm, TaL 80 mm; USNM 587919). The body is quite slender in males, more robust in large females, laterally compressed; head triangular, elongate, wide posteriorly, flattened in males, moderately thick in females, clearly distinct from the neck; snout elongate, distinctly flattened, rounded seen from above, angular and obliquely truncated in profile view, with a distinct canthus rostralis; nostril centered in the nasal scale; size of the eye average, 0.7–0.9 times the distance between the lower margin of the eye and the border of the upper lip; tail average to long, progressively tapering and distinctly prehensile; TaL/TL 0.177–0.230 in males, 0.136–0.161 in females.

Generally 23-21-15 dorsal scale rows (50%), less frequently 21-21-15 (30%) or 22-21-15 (20%) dorsal scale rows; 154–176 distinctly keeled ventrals (159–170 in males, 154–176 in females); scales of the first dorsal scale row smooth and not enlarged; subcaudals sexually dimorphic with slight overlap, 66–74 in males, 57–66 in females; total number of VEN+SC: 222–242; cloacal plate entire.

The head scalation is as described for the holotype, with the following variation: internasals separated by one small scale in all examined specimens; four or five canthal scales bordering the canthus rostralis between the internasal and corresponding supraocular; one supraocular on each side, entire and rather narrow, about 0.7–0.9 times as wide as the internasals, indented on its inner margin by the upper head scales; cephalic scales juxtaposed, flat and smooth; 9–14 (10–11 in most examined specimens) cephalic scales on a line between supraoculars; occipital scales rhombohedral, distinctly obtusely keeled in males, weakly keeled or even smooth in females; temporals generally moderately but distinctly keeled in most males (71.5%), sometimes smooth (28.6%); smooth in all examined females; 9–11 supralabials; third supralabial the longest and highest, rather tall, in contact with the subocular or separated from this latter scale by one scale; fourth supralabial separated from the subocular by one scale in all examined specimens; fifth supralabial smaller than the fourth one, separated from the subocular by one or two scales of similar size; 11–14 (generally 12 or 13) infralabials, those of the first pair in contact with each other.

(Figs 10–11). In life or in freshly preserved animals, the body is uniform bright green, grass-green or emerald-green (in preservative, the general background color remains green or turns to bluish-green or brown); dark, transversal interstitial crossbands faint in males, absent in females; white, elongate vertebral dots present in males; in males, the broad, bicolored ventrolateral stripe is as described above, bright red or fire-red on its lower part, on most scales of the first dorsal scale rows, white above, on scales of the second dorsal scale row and on the upper posterior part of scales of the first row, always present and conspicuous; in females, the ventrolateral stripe is absent (83.3%) or sometimes present as thin white line (16.7%). The tail is of the same green color than the dorsum, with above and on the sides a series of 15–25 large, bright rusty-red blotches, subrectangular or hexagonal, crossing the vertebral line of the tail, either distinct throughout the tail or progressively fused together posteriorly as an irregular stripe.

The dorsal surface of the head and the temporal region are uniform bright green or deep green like the body; the sides of the head below the eye, are more or less distinctly paler than the dorsal surface of the head, namely usually pale yellowish-green or pale green; in males, the vivid, broad, bicolored postocular streak associating a thin and white ventral part with a broad and bright red, rusty-red dorsal part, is always present (Fig. 10C, D, G, H); in females, the postocular streak is usually absent or present as a thin, faint white line. The chin and throat are pale yellowish-green or pale sea-green, uniform or with some darker green areas. The eye is bright red, fire-red, or deep red in specimens of both sexes.

The venter is uniformly pale green or pale yellowish-green; tips of ventrals usually green like the venter but we saw a male, especially colorful, in which the bright red part of the ventrolateral stripe also extended onto the outer parts of ventral plates. The ventral surface of the tail is like the venter with the tips of the red dorsal blotches extending onto the outer parts of some subcaudal scales; posterior part of the tail mixed rusty-red and green.

Trimeresurus tenasserimensis is distinguished from T. lanna (described above) by having: (1) lower max TL in both sexes (736 mm in males, 532 mm in females vs. 815 mm in males, 854 mm in females); (2) slightly higher total number of VEN+SC in males (226–242, x̄ = 235.3 vs. 213–241, x̄ = 227.8 in males; p = 0.018); (3) a slightly lower ratio ED/SnL in males (0.39–0.62, x̄ = 0.50 vs. 0.46–0.66, x̄ = 0.58 in males; p = 0.05); (4) elongate, white vertebral spots generally present in juvenile and subadult specimens in both sexes (vs. absent).

Trimeresurus tenasserimensis is distinguished from T. popeiorum by having: (1) lower max TL in both sexes (736 mm in males, 532 mm in females vs. 925 mm in males, 884 mm in females); (2) slightly lower ratio TaL/TL in females (0.136–0.161, x̄ = 0.152 vs. 0.14–0.19, x̄ = 0.166; p = 0.017); (3) bicolor postocular streak in males very broad, covering 2–3 temporal scales (vs. thin, covering 1–2 temporal scales); (4) temporals strong keeled in males (vs. feebly keeled). Furthermore, T. tenasserimensis is widely separated from T. popeiorum on a geographical basis as this latter species inhabits only northeastern India, Nepal, Bhutan, Bangladesh, southwestern China, and northern Myanmar. Moreover, the ranges of both species are separated by that of T. lanna.

Trimeresurus tenasserimensis is distinguished from T. nebularis by having: lower max TL in both sexes (736 mm in males, 532 mm in females vs. 1002 mm in males, 948 mm in females; (2) higher total number of VEN+SC in both sexes (226–242, x̄ = 235.3 vs. 210–218, x̄ = 214.0 in males; p = 0.02; 222–242, x̄ = 231.0 vs. 197–210, x̄ = 205.6 in females; p = 0.01); (3) eye bright or deep red in both sexes vs. usually green; (4) bicolored postocular streak present in males vs. absent; (5) ventrolateral streak present in females vs. absent; (6) ventral color green vs. usually yellowish in both sexes.

Trimeresurus tenasserimensis differs from T. phuketensis by having: (1) higher max TL in males (736 mm vs. 640 mm), but lower max TL in females (532 mm vs. 748 mm); (2) lower ratio TaL/TL in both females (0.14–0.16, x̄ = 0.15 vs. 0.17–0.18, x̄ = 0.17; p = 0.003); (3) higher total number of VEN+SC in males (226–242, x̄ = 235.3 vs. 242–249, x̄ = 246.4; p = 0.004); (4) eye color deep red in both sexes vs. copper; (5) no dorsal crossbands vs. irregular, conspicuous reddish-brown crossbands usually present.

Lastly, T. tenasserimensis can be further differentiated from the five subspecies of T. sabahi as follows:

– from T. s. barati by having: (1) higher total number of VEN+SC in both sexes (226–242, x̄ = 235.3 vs. 208–225, x̄ = 217.7 in males; p = 0.0006; 222–242, x̄ = 231.0 vs. 201–219, x̄ = 207.0 in females; p = 0.001); (2) 21 dorsal scales rows at midbody vs. 17–19; (3) eye color deep red in both sexes vs. deep orange; (4) bicolored postocular streak present in males vs. absent;

– from T. s. buniana by having: (1) lower total number of VEN+SC in males (226–242, x̄ = 235.3 vs. 246–250, x̄ = 248.0; p = 0.02); (2) lower number of cephalic scales in both sexes (9–11, x̄ = 10.0 vs. 11–12, x̄ = 11.7 in males; p = 0.03; 11–13 vs. 14 in female); (3) eye color deep red in both sexes vs. copper; (4) dorsum uniform bright green vs. dark bluish-green or verdigris with conspicuous reddish-brown or violet, irregular crossbands; (5) bicolored postocular streak, red and white, in males vs. postocular streak reddish-brown;

– from T. s. fucatus by having: (1) lower max TL in both sexes (736 mm in males, 532 mm in females vs. 834 mm in males, 826 mm in females); (2) lower ratio TaL/TL in females (0.14–0.16, x̄ = 0.15 vs. 0.16–0.19, x̄ = 0.17; p = 0.002); (3) eye color deep red in both sexes vs. copper; (4) solid dorsal crossbands in males absent vs. present; (5) wide bicolor postocular streak, red and white, present in males vs. sometimes absent, or white, or thin, white with a dark red upper part;

– from T. s. toba by having: (1) lower max TL in females (532 mm vs. 798 mm); (2) slightly higher total number of VEN+SC in females (222–242, x̄ = 231.0 vs. 204–218, x̄ = 212.5; p = 0.02); (3) eye color deep red in both sexes vs. deep orange; (4) in males, ventrolateral stripe bicolored, wide, vs. white and thin.

– from T. s. sabahi by having: (1) slightly lower ratio TaL/TL in females (0.14–0.16, x̄ = 0.15 vs. 0.17–0.18, x̄ = 0.18; p = 0.02); (2) higher total number of VEN+SC in both sexes (226–242, x̄ = 235.3 vs. 216–226, x̄ = 222.0 in males; p = 0.01; 222–242, x̄ = 231.0 vs. 212–217, x̄ = 214.7 in females; p = 0.02); (3) bicolored postocular streak present in males vs. absent.

(Fig. 1). Currently, T. tenasserimensis is likely to be restricted to the northern part of the Isthmus Kra, in the Tennasserim Range of peninsular Myanmar and Thailand. Based on our data, we establish the distribution of this species as follows: Peninsular Thailand (Ratchaburi, Kanchanaburi, Phetchaburi, and Prachuap Khiri Khan provinces), and southeastern Myanmar (Tanintharyi Region).

Trimeresurus tenasserimensis inhabits a wide variety of habitats, from lowland bamboo forest and dry evergreen forest to submontane forest, at elevations from 200–1500 m. This pitviper usually occurs near small streams or in wet habitats. In Lampi Marine NP., Tanintharyi Region, Myanmar, this species was found coiled in trailside vegetation (ca. 50 cm above the ground) in undisturbed tropical evergreen forest on ridgeline (Platt et al. 2018). Arboreal and nocturnal, it is mostly found hanging above streams, sometimes at 6–8 m above water. Breeding starts around August, and newborn snakes appear around mid-December (personal observations). Diet in the wild is mostly based on frogs and geckos such as Cyrtodactylus cf. oldhami (Theobald). Large females sometimes feed on small rodents (personal observations).

Further research is required to clarify the extent of the distribution, population trends and conservation status of the new species. Trimeresurus tenasserimensis is distributed over a relatively small region, but inhabits several protected areas. Across its range, the new species is quite common. Thus, we tentatively suggest that T. tenasserimensis be assessed as Least Concern (LC) following the IUCN‘s Red List categories (IUCN Standards and Petitions Committee 2019).