Oligodon arenarius : Vassilieva (2015: 212). Holotype: “Binh Chau–Phuoc Buu Nature Reserve, Xuyen Moc District, Ba Ria–Vung Tau Province, southern Vietnam, coordinates 10°29′46″N, 107°27′54″E, elevation 5 m a.s.l.”

MNHN-RA 1928.0119, an adult male from “Annam: Pointe Lagan (sur les dunes)” (now Cape Lagan or Mui La Gan, Binh Thuan Province, Vietnam, 11°10′N, 108°42′E), collected by M. Pierre Chevey on 17 May 1926 (Chevey 1927). Lost fide Nguyen et al. (2009) and Uetz et al. (2019).

ZFMK 88885, an adult male from “coastal dune area about 1 km from the coast line”, Mui Ne, Binh Thuan Province (10°57’N, 108°19’E), collected by Peter Geissler on 27 April 2009.

All from Vietnam. NHMUK 1938.8.7.39 male, “Nha Trang, S. Annam” (now Nha Trang, Khanh Hoa Province), collected by M. A. Smith; NHMUK 1969.1854 female, “Saigon” (now Ho Chi Minh City); NHMUK 1969.1855–1856 two females, “near Na Thrang” (now Nha Trang, Khanh Hoa Province); ZMMU Re-11561 female, Nui Chua National Park, Ninh Thuan Province (11°46′N, 109°09′E), collected by V. V. Bobrov on September 10, 2003; ZMMU Re-13857 male, Hon Tre Island, Khanh Hoa Province (12°11′N, 109°17′E), collected by A. B. Vassilieva on 2 January 2011; ZMMU Re-15136 female, paratype of O. arenarius, Binh Chau–Phuoc Buu Nature Reserve, Binh Chau, Xuyen Moc District, Ba Ria–Vung Tau Province (10°29′N, 107°27′E), collected by A. B. Vassilieva on 9 November 2010; ZMMU Re-14502 male, paratype of O. arenarius, Binh Chau–Phuoc Buu Nature Reserve, Binh Chau, Xuyen Moc District, Ba Ria–Vung Tau Province (10°32′N, 107°28′E), collected by A. B. Vassilieva on 9 November 2010; ZMMU Re-14503 male, holotype of O. arenarius, Binh Chau–Phuoc Buu Nature Reserve, Binh Chau, Xuyen Moc District, Ba Ria–Vung Tau Province (10°29′N, 107°27′E), collected by A. B. Vassilieva on 14 November 2014; ZMMU Re-14504 female, paratype of O. arenarius, Binh Chau–Phuoc Buu Nature Reserve, Binh Chau, Xuyen Moc District, Ba Ria–Vung Tau Province (10°29′N, 107°27′E), collected by A. B. Vassilieva on 16 November 2014; VNMN 04274 female, paratype of O. arenarius, Binh Chau–Phuoc Buu Nature Reserve, Binh Chau, Xuyen Moc District, Ba Ria–Vung Tau Province (10°30′N, 107°28′E), collected by N. A. Poyarkov on 10 July 2012; UNS05001–05002, two males, Mui Ne, Binh Thuan Province (10°58′N, 108°20′E), collected by P. Geissler on 4–5 March 2011 respectively (Geissler et al. 2011); DTU 532, female, Mui Ne, Binh Thuan Province, collected by Hieu Minh Pham on July 2019; ITBCZ 6884, male, on the coastal road near Ho Coc Resort, Xuyen Moc District, Ba Ria–Vung Tau Province, Vietnam (10°29′49.2″N, 107°27′36″E), collected on 15 June 2019 by Sang Ngoc Nguyen and Vu Dang Hoang Nguyen (Nguyen et al. 2021); ZMMU Re-16804–16806, three males, previously identified as O. arenarius, Binh Chau–Phuoc Buu Nature Reserve, Binh Chau, Xuyen Moc District, Ba Ria–Vung Tau Province (10°30′N, 107°28′E), collected by N. A. Poyarkov and P. V. Yushchenko on 24–28 December 2020; ZMMU Re-16807–16809, three males, Phan Thiet City, Binh Thuan Province (10°56′N, 108°17′E), collected by Hieu Minh Pham on October 2020; ZFMK 88884, female from Mui Ne, Binh Thuan Province (10°56′N, 108°17′E), collected by Peter Geissler on 27 April 2009 .

An Oligodon species distinguished from all other members of the genus by having the following morphological characters: 1) medium body size in adults (TotalL 245–510 mm); 2) a very long relative tail length, especially in males (TAILR 25.4%–37.3% in males, 14.0–19.6% in females; SCR 29.3%–38.7% in males, 20.0%–25.0% in females); 3) head small and slightly spade-shaped, comparatively short and wide (HW/HL 0.53–0.90); 4) dorsal scale rows usually 17-17-15, rarely 18 scale rows anteriorly and 16 rows at midbody; 5) ventral scales 131–152 in males, 139–169 in females, with significant sexual dimorphism; 6) subcaudals 60–94 in males, 36–48 in females with significant sexual dimorphism; 7) total body scales trending towards sexual dimorphism, 191–243 in males, 178–214 in females (178–243 in both sexes); 8) cloacal plate single; 9) 1 preocular and usually 2 postoculars (rarely 1 postocular); 10) nasal scale divided, loreal scale and presubocular condition variable (can be present or absent); 11) usually 8 supralabials (rarely 7), with the 4^th and 5^th scales in contact with the orbit (occasionally just the 4^th supralabial contacting the orbit); 12) usually 9 infralabials (rarely 8 or 10), anterior 4^th and 5^th infralabials contacting the first pair of chin shields; 12) internasals present, separate from prefrontals; 13) temporal scale formula 1+2 (rarely 1+3); 14) maxillary teeth 9–12, with posterior teeth enlarged and blade-like; 15) hemipenes deeply bilobed, retracted organ reaching the 29^th subcaudal in-situ, without spinous calyces and with flounced structures across lobes; 16) dorsal color pattern variable, ochre brown or orange–brown above with light dark brown reticulations and a dark V-shaped nuchal collar present; 17) ventral color pattern pale gray and immaculate.

We compare O. macrurus to other members of Oligodon found in the O. cyclurus-taeniatus species group (Green et al. 2010; similar in composition to the O. cyclurus and O. taeniatus groups fide Smith 1943). We divide the O. cyclurus-taeniatus species-group into two further subgroups, the O. cyclurus subgroup and the O. taeniatus subgroup, since they are apparently closely related and are recovered sister to one another in the same clade (Leviton 1962; Green et al. 2010; Nguyen et al. 2020; Nguyen et al. 2022; present paper). It differs from all other species of the genus by the unique combination of the following characters: usually 17-17-15 dorsal scale rows; 134–169 ventrals; expressed sexual dimorphism in relative tail length (TAILR), varying from 14.0% in females and 37.3% in males and the number of subcaudals, varying from 36 in females to 94 in males; an entire cloacal plate; divided nasal scale; a deeply bilobed hemipenis without spines or obvious myoectases, and the absence of a specific dorsal coloration pattern (i.e., large blotches or bright longitudinal stripes). Additional comparisons between the Oligodon native to Vietnam can be found in Table 8.

Members of the informal O. taeniatus subgroup (sensu David et al. 2008b; Smith 1943; Vassilieva 2015) possess deeply bilobed hemipenes with myoectases visible when retracted, whereas O. macrurus does not possess myoectases on the retracted hemipenes. Furthermore, the O. taeniatus subgroup all have a dorsum with vertebral stripes or blotches (vs. dorsum without vertebral stripes or large blotches, reticulate and only with crossbars anteriorly). Oligodon barroni has a red venter with black quadrangular spots (vs. immaculate venter); Oligodon deuvei David, Vogel and Van Rooijen, 2008 by having the 3^rd and 4^th supralabials contacting the eye (vs. usually 4^th and 5^th contacting eye) and a venter with black rectangular blotches (vs. immaculate venter); Oligodon moricei David, Vogel & Van Rooijen, 2008 has a higher number of ventrals (175 vs. 131–169) and a venter with grayish–brown blotches (vs. immaculate venter); Oligodon mouhoti (Boulenger, 1914) and O. pseudotaeniatus David, Vogel & Van Rooijen, 2008 have a venter with black quadrangular spots (vs. immaculate venter); and O. taeniatus has 19 midbody scale rows (vs. usually 17).

The O. cyclurus subgroup (sensu David et al. 2008a; Green et al. 2010; Smith 1943; Vassilieva 2015) have deeply bilobed hemipenes without spines or myoectases, which greatly resemble the morphology observed in O. macrurus. Compared to O. macrurus, O. chinensis has a higher number of ventrals (170–206 vs. 131–169) and has a dorsum with large dark brown blotches (vs. dorsum without blotches); Oligodon condaoensis Nguyen et al., 2016 has a higher number of ventrals (168–176 vs. 131–169) and has a dark gray dorsum (vs. dorsum light brown, ochre brown or orange–brown); O. culaochamensis Nguyen et al., 2017 has a higher number of ventrals (167–182 vs. 131–169), 2+2 temporals (vs. 1+2 temporals), and a dorsum with large dark brown blotches (vs. dorsum without blotches), O. cyclurus has 19 midbody scale rows (vs. usually 17), a higher number of ventrals (167–197 vs. 131–169); Oligodon fasciolatus (Günther, 1864) has 21–23 midbody scale rows (vs. usually 17); O. formosanus has 19 anterior scale rows (vs. no more than 18); Oligodon huahin Pauwels et al., 2017 has a slightly higher number of ventrals (166–173 vs. 131–169), only 8 infralabials (vs. usually 9, rarely 8–10) and a distinct gray dorsum (vs. dorsum mostly ochre brown); Oligodon juglandifer (Wall, 1909) has 19 midbody scale rows (vs. usually 17), 7 supralabials (vs. usually 8, rarely 7), 8 infralabials (vs. usually 9, rarely 8–10), and a higher number of ventrals (162–208 vs. 131–169); Oligodon kampucheaensis Neang, Grismer & Daltry, 2012 has 15 midbody scale rows (vs. usually 17) and 8 infralabials (vs. usually 9, rarely 8–10); Oligodon kheriensis Achardji & Ray, 1936 has 19 midbody scales (vs. usually 17), and a bright red dorsum (vs. dorsum light brown, ochre brown or orange–brown); O. ocellatus has 19 midbody scale rows (vs. usually 17); and Oligodon saintgironsi David, Vogel & Pauwels, 2008 has a higher number of ventrals (166–184 vs. 131–169) and 2+2 temporals (vs. 1+2). A few species share a close relationship to the O. cyclurus and O. taeniatus subgroup, but are basal phylogenetically and are compared here. Oligodon annamensis has 13 dorsal scale rows (vs. usually 17-17-15), 6 supralabials and 6 infralabials (vs. usually 8 supralabials and 9 infralabials), and has a dorsal coloration with orange crossbars (vs. dorsum with dark brown crossbars only anteriorly); O. rostralis has 15 dorsal scales (vs. usually 17-17-15), 6 supralabials and 6 infralabials (vs. usually 8 supralabials and 9 infralabials) and a dorsal coloration with dark blotches (vs. dorsum with no blotches); O. octolineatus, which is seems to be the sister taxa to the informal O. taeniatus-cyclurus species group, usually has 6 (rarely 5–7) supralabials (vs. usually 8, rarely 7), 2+2 temporals (vs. 1+2), and a dorsum with bright longitudinal stripes (vs. dorsum without stripes).

An adult male specimen in good condition, found DOR. Partial incision anteriorly and a longer incision made posteriorly before the cloaca. SVL 279 mm, TailL 146 mm (TotalL 425 mm). HeadL 12.5 mm, HeadW 7.9 mm, SnoutL 3.9 mm, EyeD 1.9 mm, FrontalL 3.5 mm, FrontalW 2.9 mm, IOD 4.4 mm, IND 3.3 mm. TailLR 34.4%, HeadW/L 0.63, SnoutL/HeadL 0.31, EyeD/SnoutL 0.49, EyeD/HeadL 0.15, FrontalL/W 1.21, IND/IOD 0.21, IOD/HeadW 0.56. Body elongated but somewhat flattened due to preservation state, slightly robust anteriorly and at midbody; head ovoid, slightly distinct from neck; snout narrowing in dorsal view, depressed and truncate towards the rostral in dorsolateral view; snout tip terminating past lower jaw; eyes moderately-sized with a round pupil; nostrils pointed laterally; mouth flat, curving slightly posteriorly; tail long, consistent in diameter until the posterior half where it tapers gradually to a sharp terminal scute.

Rostral distinctly enlarged and truncate laterally, wider than high and triangular in dorsal view, partially separating internasals; posterior scale suture of rostral with internasals “deep-V” shaped, vertex of rostral rising far onto the dorsal surface of the head in-line with nostrils as a narrow obtuse angle (~97º); internasals subrectangular, longer than wide, internasal suture shorter than prefrontal suture, anterior border with rostral and nasal concave; prefrontals subpentagonal, longer than wide, wider than internasals; frontal subpentagonal and shield shaped; length of frontal longer than prefrontals and internasals; anterior suture of frontal bordering prefrontals straight but somewhat indented; eyes placed posterior relative to the anterior edge of the frontal; angle formed by the sutures producing the posterior vertex of the frontal narrowly obtuse (~94º); supraoculars subrectangular, longer than wide, narrower anteriorly than posteriorly; length of frontal longer than supraoculars; parietals subpentagonal, slightly longer than wide, width of each scale wider than length of parietal suture; length of parietal scale slightly longer than length of frontal; parietal suture shorter than length of frontal; anterior parietal angle formed by the sutures between the parietal/frontal and the suture between the supraocular/parietal an obtuse angle (~122º) with the lateral ray of the angle pointing posterolaterally; nasal scale subrectangular, longer than wide and fully divided; loreal scale present (1/1), also subrectangular, slightly longer than wide, around half the size of nasal; supralabials 8/8, with the 4^th and 5^th scales in contact with the orbit; 7^th supralabials largest, 1^st supralabial smallest; preoculars 1/1; presuboculars (1/1), smaller and less wide than preocular; postoculars 2/2, uppermost postocular larger in size on left side, bottommost postocular on right side slightly wider; temporal scale formula 1+2, uppermost posterior temporal longer with 6/6 scales surrounding scale; infralabials 9/9, first pair contacting eachother; 4/4 infralabials contacting the first pair of chin shields; 5^th infralabial largest, 2^nd infralabial smallest; mental subtriangular, wider than long; small mental groove present starting below the mental scale where the first pair of infralabials contact, then terminating at the level of the posterior chin shields; anterior pair of chin shields longer than the posterior pair; anterior chin shields slightly wider than posterior chin shields.

Dorsal scale rows 17-17-15, smooth throughout; reduction from 17 to 15 scale rows occurring on 87^th ventral on either side; ventral scales 142, subcaudals 87, cloacal plate divided (total body scales 230); subcaudal ratio 38.0%. Maxillary teeth not counted (see General description and variation for details on dentition). The hemipenes were partially everted in an unilobed state, but not fully prepared. The base of the organ is relatively naked with a few flounces present and the sulcus spermaticus partially visible.

In preservative, dorsum light brown with small dark brown or black reticulations formed by dark edges along the dorsal scales; dorsal reticulations concentrated anteriorly and at midbody, whereas the posterior portion of the dorsum is mostly immaculate; vertebrally a series of crossbars, beginning anteriorly as a pair of dark brown spots and continuing as narrow black bars around one or one-half a scale wide, slowly fading in size by midbody and continuing up to the tail as small subrectangular spots. The head is brown dorsally with a poorly defined gray–brown ocular bar edged posteriorly with black, extending across the eyes then meeting at the prefrontals and anterior portion of the frontal; a grayish brown and black-edged temporal streak present on each side of the head starting from the medial portion of each parietal through the posterior temporals and supralabials before dissipating at the gular region. The frontal and parietals have small dark vermiculations present on the scales, along with a dark brown irregularly-shaped spot on the posterior vertex of the frontal and on the suture of the parietals. Along the nape there is a large gray–brown V-shaped nuchal chevron beginning at the posterior end of the parietals around six dorsal scales in length before forking at the nape as a dark brown streak terminating along the flanks of the first two dorsal scale rows. The remainder of the head, including the labial and ventral regions, are beige and mostly immaculate. Ventral surface light brown to beige, immaculate without any spotting or makings. Dorsal surface of tail light brown, immaculate laterally and vertebrally between two dark brown longitudinal stripes originating from the dorsum and extending posteriorly from the cloaca as a pair of stripes extending across the tail to its tip. Ventral surface of tail beige and immaculate.

All examined specimens agree with the original description of the lost holotype and the newly designated neotype (ZFMK 88885). A summary of morphological data for all presently known specimens of O. macrurus is presented in Table 7. In all specimens, body elongated and semi-cylindrical, slightly robust anteriorly and at midbody; head ovoid, slightly distinct from neck; snout narrowing in dorsal view, depressed towards the rostral in dorsolateral view; snout tip terminating past lower jaw; eyes moderately-sized with a round pupil; nostrils pointed laterally; mouth flat, curving slightly posteriorly; tail long in males, consistent in diameter until the posterior half before tapering gradually to a sharp terminal scute; in females, tail still elongate but much shorter than males, tapering gradually starting anteriorly from the cloaca to the terminal scute.

SVL 210–320 mm (212–320 mm in males, 210–319 mm in females); TailL 35–190 mm (72–190 mm in males, 35–75 mm in females); TotalL 245–510 mm (284–510 mm in males, 345–385 mm in females). The largest specimen is an adult male (UNS 5001) with a SVL of 320 mm and TailL of 190 mm (TotalL 510 mm; TailLR 37.3%). HeadL 8.9–14.3 mm, HeadW 6.1–10.3 mm, SnoutL 3.1–4.9 mm, EyeD 1.5–2.1 mm, FrontalL 2.8–4.2 mm, FrontalW 2.3–3.4 mm, IOD 3.6–5.6 mm, IND 2.1–3.8 mm; TailLR 14.0%–37.3% (25.4%–37.3% in males, 14.0–19.6% in females), HeadW/L 0.53–0.90, SnoutL/HeadL 0.30–0.40, EyeD/SnoutL 0.35–0.52, EyeD/HeadL 0.13–0.18, FrontalL/W 1.10–1.35, IND/IOD 0.44–0.73, IOD/HeadW 0.48–0.64. Rostral distinctly enlarged and truncated laterally, wider than high, triangular shaped in dorsal view, partially separating internasals; posterior scale suture of rostral with internasals “deep-V” shaped; internasals subrectangular, longer than wide, internasal suture longer than prefrontal suture, anterior border with rostral and nasal concave; prefrontals subpentagonal, longer than wide, wider than internasals; frontal subpentagonal and shield shaped, length longer than prefrontals; anterior suture of frontal bordering prefrontals broad, either concave or straight; eyes placed posterior to the anterior margin of frontal; angle formed by the sutures producing the posterior vertex of the frontal a narrow obtuse angle; supraoculars subrectangular, longer than wide, length of frontal longer than supraoculars; parietals subpentagonal, slightly longer than wide, width of each scale wider than length of parietal suture; length of each parietal scale equal or slightly longer than length of frontal, but length of the parietal suture shorter than length of frontal; anterior parietal angle formed by the sutures between the parietal/frontal and the suture between the supraocular/parietal a broad obtuse angle with the lateral ray of the angle pointing posterolaterally. Nasal scale subrectangular and longer than wide, fully divided; loreal scale condition variable, 1/1 or 0/0 (0/1 in two specimens); when present, loreal square or subrectangular-shaped, slightly longer than wide, around half the size of nasal; supralabials usually 8/8 (7/7 in one specimen, 7/8 in four specimens, 8/7 in one specimen), with the 4^th and 5^th scales in contact with the orbit (3^rd and 4^th scale in contact on one side in one specimen, just the 4^th scale in contact on both sides in one specimens, on one side in four specimens); 7^th supralabial largest, 1^st supralabial smallest; preoculars 1/1; presubocular usually present (1/1) but sometimes absent (0/0 in five specimens), when present smaller than preocular; postoculars usually 2/2 (rarely 2/1 in one specimen, 1/2 in two species, 1/1 in one specimen), uppermost postocular usually larger in size when two scales are present (occasionally the bottommost postocular is wider); temporal scale formula 1+2, with one specimen having 2/3 posterior temporals; infralabials usually 9/9 (rarely 8/8 in two specimens, 8/9 in two specimens, 7/9 in one specimen, 10/10 in one specimen); first pair of infralabials in contact with each other; usually 4/4 or 5/5 infralabials contacting the first pair of chin shields, one specimen with 4/5 and another with 3/4 infralabials in contact; 5^th infralabial largest, 2^nd infralabial smallest; mental subtriangular, wider than long; small mental groove present, starting below the mental scale where the first pair of infralabials contact and extending until the posterior chin shields; length of anterior pair of chin shields longer than posterior pair; anterior chin shields slightly wider than posterior chin shields.

Dorsal scale rows 17-17-15 (rarely 18-17-15 in two specimens, or 17-16-15 in two specimens), smooth throughout; reduction from 17 to 15 scale rows occurring on ventrals 71–98; ventral scales 131–169 (131–152 in males, 139–169 in females); subcaudals 36–94 (60–94 in males, 36–48 in females); total body scales 178–243 (191–243 in males, 178–214 in females); subcaudal ratio 20.0%–38.7% (29.3%–38.7% in males, 20.0%–25.0% in females). Maxillary teeth 9–12, all blade-like, with the posterior two or three greatly enlarged. Vassilieva (2015) provided lower tooth counts ranging from 6–9 for the type series of O. arenarius, however our examination of the maxilla using µCT-scanning revealed that the number of teeth is likely higher by at least three. These differences can reflect the difficulties of counting dislodged teeth on the maxilla, especially when the gum layer can obscure views of the upper jaw when examined carelessly. Significant sexual dimorphism was observed in the number of ventrals, subcaudals, TailLR and subcaudal ratio (Table 5). In general, the tails of male specimens are much longer and less tapered in female O. macrurus and are some of the longest tails out of any members of the genus Oligodon. The hemipenes are deeply bilobed in both retracted and everted specimens examined, in-situ extending from the 25^th to 29^th subcaudal (fide. Smith 1943 and our own data); myoectases absent on retracted organ. When partially everted the hemipenis may appear unilobed with small apical flounces. Fully everted, the organ is obliquely flounced until the point of bifurcation, which occurs at the first fourth or third of the organ; lobes calyculate and without spines, becoming smoother posteriorly; lobes terminating as blunt apices; base of organ nude without ornamentation; sulcus spermaticus bifurcate (forked), extending straight from the base and then dividing at the point of bifurcation and continuing along the lobes to the apices.

Most O. macrurus specimens exhibit a similar color pattern, but some intraspecific variation is present. The dorsum in-life is ochre brown, light brown or orange–brown, and all specimens have small dark-brown reticulations across the body formed by dark edges along the dorsal scales; dorsum usually darker vertebrally, occasionally with two darker lines creating a small weakly-distinguished and dusky pair of longitudinal stripes, especially distinct on the tail. The head is marked with a dark ocular bar that extends across the eyes and meeting at the prefrontals and anterior portion of the frontal, a temporal streak on each side of the head starting from the parietals to the supralabials and flanks, and a V-shaped nuchal chevron. The ventral surface is plain white or beige and without any spots or blotches. Three main color variants can be observed, well correlated with geographic location. The first is a “north” form observed in specimens from Khanh Hoa Province (NHMUK 1938.8.7.39, 1969.1855–56, ZMMU Re-13857). In these individuals, the ocular bar is indistinguishable or barely distinguishable from the rest of the head, the temporal streak is faint (and reduced to the lower half of its length in NHMUK 1938.8.7.39 and NHMUK 1969.1855), and the narrow V-shaped nuchal collar is only 2–4 dorsal scales in length and of equal width. There are no dark markings on the frontal scale, and the dorsum has small dark brown crossbars starting posterior to the nuchal collar fading by midbody to small indistinguished reticulations. The second “middle” form (Figs 5, 6A) is found in specimens from Binh Thuan Province (ZFMK 88885 [neotype], UNS05001-05002, the lost holotype and ZMMU Re-14502), where the ocular bar is dark brown and fairly distinct (faint in ZMMU Re-14502, but this is probably due to preservation conditions), the temporal stripe is also distinct (divided on two parts in ZMMU Re-14502), and V-shaped nuchal collar is more elongated (4–6 scale rows long) and widest medially before narrowing laterally, including the posterior portion of the parietals. The frontal spot is present and usually distinct (with exception of the lost holotype), and the remainder of the dorsum has several paravertebral pairs of narrow spot-shaped crossbars on the anterior half of the body. The “south” form (Figs 6B–D) occurs in specimens from Ba Ria–Vung Tau (ZMMU Re-11561, Re-14503–04 and VNMN 04724) and Ho Chi Minh City (NHMUK 1969.1854), where the ocular bar is distinct and dark-brown (faint in NHMUK 1969.1854, but again probably due to conditions of preservation), the temporal stripe is also usually distinct (faint in NHMUK 1969.1854) and the V-shaped nuchal collar is 4–5 scales long but wide both medially and on the flanks and fused with markings on the frontal. The dorsum has paravertebral pairs of dark-brown spots present mostly anteriorly (ZMMU Re-11561, R-14503, NHMUK 1969.1854) or throughout the whole body (ZMMU Re-14504 and VNMN 04724). Oblique lateral streaking on the dorsum are also displayed on a few specimens (with exception of NHMUK 1969.1854). The specimen ZMMU Re-15136 (from Ba Ria–Vung Tau) stands out for its saturated orange color in life and black speckling evenly distributed along the body. However, it seems to share more in common with the “south” form. The presence of more or less distinct vertebral stripe, especially on the tail, seems to be the common feature for all three groups with exception of ZMMU Re-15136 and, apparently, the lost holotype. It should be noted that the color present in the “south” form includes specimens previously referred to O. arenarius, as well as specimens identified as O. macrurus.

Figure 5. 

Photographs of the preserved neotype of Oligodon macrurus (ZFMK 88885), an adult male specimen. A dorsal and B ventral views of the whole specimen, and C dorsal D ventral, E right lateral and F left lateral views of the head. Scale bars for A–B represent 10.0 mm, and scale bars for C–F represent 5.0 mm. All photographs taken by Morris Flecks, used with permission.

Figure 6. 

Living specimens of Oligodon macrurus sensu stricto from various locations in southern Vietnam. A adult male ZMMU Re-16807 from Mui Ne, Binh Thuan Province, representing the ‘middle’ color phase; B adult female VNMN 04724 (formerly ZMMU NAP-03884) from Binh Chau-Phuoc Buu Nature Reserve; C and D two adult males ZMMU Re-16804 and ZMMU Re-16805 from Binh Chau-Phuoc Buu Nature Reserve, Ba Ria-Vung Tau Province, B–D formerly identified as Oligodon arenarius. Photographs taken by Hieu Minh Pham A and Nikolay A. Poyarkov Jr. B–D.

The description of the skull of Oligodon macrurus is based on 3D reconstructed µCT-scans of two specimens: ZMMU Re-13857 and ZMMU Re-14502 (one of the paratypes of O. arenarius); (Fig. 7). The skull of O. macrurus is short, rounded, and well ossified. The snout is composed of the premaxilla, nasals, septomaxillae and vomers. The circumorbital bones include the prefrontals and postorbitals. Premaxilla single, flat and curved anteriorly, representing the front tip of the snout; deeply wedged in the space between and beneath the septomaxillae and the nasals. Nasals spatulate-shaped and sharpened at the tip with an S-shaped profile; left and right articulated nasals form median septum between the nasal cavities covering them dorsally; ventrally nasals form a posterior process lying between the anterior edge of the frontals. Septomaxillae paired and plate-like, in contact medially, forming the floor of nasal cavity, partially fused with vomers; conchal processes of septomaxillae well defined; each septomaxilla in contact with nasal septum medially and form a posterior process contacting anterior edge of frontals through the prokinetic joint. Vomers toothless, positioned beneath and behind the two septomaxillae and forming a pair of spherical fenestrae, in which lies the vomeronasal organs, opened by paired orifices into the buccal cavity. Prefrontals on either side of the head, block shaped and obliquely positioned forming the anterior edge of the orbit; dorsally prefrontals in broad contact with the anterolateral surface of the frontals, ventrally in loose contact with the maxillae. Left and right postorbitals articulate with the anterolateral surfaces of the parietal and form the dorsoposterior boundary of each orbit.

Figure 7. 

Three-dimensional CT reconstruction of the skull, lower jaw and maxilla of A–C Oligodon arenarius (ZMMU Re-14502, paratype) and D–F O. macrurus (ZMMU Re-13857) showing A1, D1 dorsal, A2, D2 lateral, and A3, D3 palatal view of the skull labial view. B, E of the lower jaw, C1, F1 labial, C2, F2 lingual, and C3, F3 dorsal view of the maxilla. Reconstructions by Elena V. Syromyatnikova.

The braincase is composed of compactly ossified bones consisting of the frontals, parietal, basisphenoid, basioccipital, prootics, supraoccipital and exoccipital; partially fused to each other forming the complete enclosure of the brain. Frontals well separate. Parietals elliptical shaped and the largest cranial element, fused together to form a single bone that dorsally roofs the braincase, bearing no elaborated crests; laterally parietal extends far down contacting the basisphenoid and the prootics. Basisphenoid and parasphenoid fused with each other, forming the posterior snout and anterior floor of braincase. Basioccipital forms the floor of the posterior portion of the brain cavity, and completes the foramen magnum creating a large and raised occipital condyle. Left and right prootics large, subrectangular ventrally and dome contoured dorsally, partly fused with the parietal and forming the anterior walls of each internal otic capsule; prootics form the anterior half of each fenestra ovalis and the posterolateral wall of the braincase. Supraoccipitals fused together to form a single bone, externally roofing the posterior brain cavity, internally expanding to form the posterior part of each otic capsule. Exoccipital forming the posterolateral wall of the braincase and part of its roof; exoccipital fused with the opisthotics, surrounding the jugular foramen and extending forward to form the posterior border of the fenestra ovalis, the entire oval foramen magnum, and a small ventral portion of the occipital condyle along with most of the basioccipital. Stapes slender, rod like, proximally enlarged and form a footplate fitting into the fenestra ovalis and distally connect to the inner surface of the quadrate at about mid length level.

The palatomaxillary arches consists of the palatine, pterygoid, ectopterygoid and maxilla. Palatines long and narrow, in contact with the prefrontal process of the maxilla laterally and pterygoid posteriorly; 7 small sized palatine teeth. Pterygoids long and slightly bent bones, narrower anteriorly, flattened posteriorly, and extend from the posterior palatines to the posterior mandible. Each pterygoid bears 10–12/9–10 teeth. Ectopterygoids flat, bifurcate anteriorly, notched posteriorly and connect the maxillae to the pterygoids. Left and right maxillae comparatively straight and posteriorly broadened due to the dorsal ridge, connected to the flattened ventral surface of the ectopterygoid by a mesial process; the maxilla medially contacts the ventral surface of the prefrontal. Each maxilla almost has no edentulous region anteriorly, and bears 10 to 12 teeth, with the posterior 2–3 enlarged and blade-like. This number of maxillary teeth agrees well with the known data for O. macrurus and differs from the data represented in the original description of O. arenarius (Vassilieva 2015).

The suspensorium contains the quadrate and supratemporal, connecting to the mandibles by elastic tissue. Supratemporals narrow, flattened, dermal elements, connected to the proximal end of quadrates and the posterolateral part of braincase by fibrous connective tissue; each is long, straight, slightly bent toward the braincase, and overlay the exoccipital and prootic. Quadrates long, widely flattened, concave dorsally with a fenestra found posterolaterally; proximal end contacting the posterolateral edge of each supratemporal; distally articulated surface of each quadrate narrow, extended transversely and directed backward. Mandibles long and connected to each other anteriorly by an elastic ligament; each composed of the compound, angular, splenial and dentary. The compound is strongly concave dorsally, narrow distally, and massive and laterally flattened proximally; prearticular crest higher than subarticular crest. Angular and splenial both triangular shaped elements that fuse at their broadest point of contact. Dentaries somewhat dorsally curved, bearing sockets for closely set 14–15/14–17 small teeth that decrease in size posteriorly.

To date O. macrurus is reliably known from five provinces in southern Vietnam (Khanh Hoa, Ninh Thuan, Binh Thuan, Ba Ria–Vung Tau and possibly the vicinity of Ho Chi Minh City), where it is only found in coastal ecosystems associated with the Mui Ne dunefields and sandy coastlines to its north and south (Fig. 1). The locality of Ho Chi Minh City (based on specimen NHMUK 1969.1854) has no precise information associated with it, and requires additional confirmation; although, suitable habitat may exist within the region. Vassilieva (2015) described aspects of behavior and habitat of O. arenarius. One specimen was found feeding on a frog Microhyla pulchra Hallowell, 1861 (Vassilieva 2015). During the collection of new material, we found specimens crossing roads at night and on the crawl in habitat during daytime searches, including a few that were recovered DOR. Based on collection records with dates, this species appears to be surface active year-round, although there is a slight increase in records between the months of November to March, corresponding with the end of southern Vietnam’s monsoon season. Habitats where O. macrurus were found include low-sloped littoral dunefields with short vegetation and ecotones with sandy clearings along lowland dipterocarp forests adjacent to dune habitats.

The specific name “macrurus” is a Greek adjective derived from the words “makrós” (μακρός) meaning “long”, and “oύrá” (οὐρά) meaning “the tail”, here latinized as -urus and thus literally denoting “long-tailed”. Common names previously attributed to this species include “Angel’s kukri Snake” (English) and “Oligodon anzhela” (Russian). The synonym O. arenarius was not given a common name during its description, although “Dune kukri snake” has sometimes been attributed, due to its epithet translating to “coast” or “dunes”. Since this species now includes this combination, we herein suggest the common name “Long-tailed kukri snake” (English), “Rắn khiếm đuôi dài” (Vietnamese), and “Dlinnohvostiy oligodon” (Russian), respectively, for O. macrurus, which directly translates to its Greek species epithet.

This species is now known specifically from seven localities across the coast of southern Vietnam. A few of these sites are found within nature reserves and other preserved tracts of land, however human development and increased tourism around these areas could pose a significant threat to this species. Geissler et al. (2011) collected two specimens as roadkill indicating that road mortality could be a potential hazard in some locations. Additionally, Vassilieva (2015) noted that the type locality of the synonym O. arenarius was highly disturbed, with surrounding areas where specimens were collected consisting of hotels and residential properties. The protection and management of dunefield habitats across the known localities of O. macrurus should be of conservation priority. Based on the assessment criteria adopted by the International Union for Conservation of Nature (IUCN), we suggest that O. macrurus should be listed as “Vulnerable” on the IUCN Red List, due to the risks associated with habitat destruction and disturbance. Additional research understanding the population dynamics and ecology of this species would greatly improve conservation efforts.