Hypsirhina innominata Morice, 1875c Coup d'Oeil sur la Fauna de la Cochinchine Francaise, 1875c:58. Type locality: from Tay-ninh, Cochin China (South Vietnam, on the Cambodian border). Holotype: MHNL 42000338.
Hypsirhina jagorii – Tirant, 1885 Notes sur les Reptiles…. De la Cochinchine et du Cambodge, p. 44.
Enhydris innominata - Smith 1929 Journal of the Natural History Society of Siam 8:49.
Enhydris innominata innominata Saint Girons 1971 Société du Science Naturelles Physiques du Maroc 51:221.
Saint Girons (1971) relegated Hypsirhina longicauda Bourret and H. smithii Boulenger to races of E. innominata. Note that elsewhere in this work E. smithii was placed in the synonymy of E. jagorii. There is little doubt that longicauda, jagorii, and innominata are closely related and that their relationships might be better expressed as races of jagorii (the oldest name of the three). Morphologically these snakes are very similar and differ from each other primarily in ventral and subcaudal counts, color patterns, degree of tail compression, and sexual dimorphism.
Etymology: The name innominata is derived from the Latin innominatus, meaning “nameless”.
Common Name: Mekong Delta Watersnake (Stuart, 2004).
Diagnosis: Enhydris innominata has 21 scale rows at midbody, an extremely low ventral count (106 -117), and a highly distinctive pattern of black lateral blotches that are scalloped-shaped that arise from the ventral surface. It can be readily separated from E. enhydris and E. subtaeniata, both of which have smooth, striated dorsal scales and elongated anterior chin shields. Both of these species also have 137 ventral scales or more. Table 11 compares it to other species within the Enhydris jagorii Group, and it is most readily confused with these close relatives which have smooth, non-striated dorsal scales and flared anterior chin shields. Enyhdris innominata has 23 - 25 scale rows on its neck, while E. chanardi has 20 - 23 (usually 21) scale rows on the neck. Additionally, E. chanardi has a higher ventral scale count (110 - 125), small dorsolateral spots that involve two or three scales; and larger, plate like temporal scales. Enhydris jagorii also has a higher ventral count (117 - 127), the same number of scale rows on the neck (23 - 25), small temporal scales, but a pattern similar to innominata. Enhydris longicauda has more ventral scales (122 - 136) and more subcaudal scales (52 - 76) than innominata which has 42 - 56 subcaudal scales.
Size: The largest female measured for this study had a total length of 655 mm, with a 50 mm tail. The largest male measured was 390 mm SVL and a damaged tail. Tails of males were 25 - 33% of the SVL; females had tails that were 18 - 24% of the SVL. The smallest measured was a male with a total length of 233 mm and a 47 mm tail. The smallest female measured was one of the syntypes that had a total length of 215 mm with a 39 mm tail.
The head is narrow and moderate in length. The eyes are dorsolateral, and their diameter is about half of the eye-nostril distance. The interocular distance is greater than the length of the frontal.
On the head the rostral scale is pentagonal and at least twice as broad as tall. It is slightly visible from above. The nasals are semi-divided and the nasal cleft touches the first labial. The internasal is single and broad, posterior to the nasals, and penetrates the nasals slightly, and it is in narrow contact with the loreals. The two prefrontals contact the loreal, the supraocular, and the preocular scales. The frontal is almost as long as the interorbital distance, and the parietals are slightly longer than the frontal. The supraocular scales are rectangular, usually single but may be divided; the preocular is usually single but may be divided, it is narrow and tall, its ventral edge is wider than the dorsal edge; and the upper postocular scale is tall and narrower than the bottom postocular scale. The single loreal contacts upper labials 1 - 3. The temporal formula is 1 + 2 + 3. The third row of temporal scales has an enlarged upper scale but the bottom two scales are indistinguishable from dorsal scales. The upper labials usually number eight, but may be seven or nine. The fourth upper labial enters the orbit. Four of 18 specimens examined had the rear upper labial divided on one side.
On the chin the lower labials number 9 - 12, 11 is the usual number; the largest can be the fourth or the sixth. Lower labials 1 - 4, 1 - 5, or 1 - 6 contact the anterior chin shields. The anterior chin shields are longer than the posterior pair and bordered by lower labials 1 - 4, 1 - 5, or 1 - 6. The posterior pair is separated by a pair of smaller scales. The gulars number 6 - 9.
On the body the anterior dorsal scale rows on the neck number 21 - 25; at mid-body number they number 20 - 23, with 21 the usual number. The syntype MHNL 42000338 has 23 rows at mid-body; Smith (1943) also noted this unusual scale count for this species. Scale rows in front of the vent are reduced to 19 or remain at 21 rows. Scales in the first row are larger than those of the second row. The dorsal scales become more ovate posteriorly and do not have ornaments. The ventral scale count range is 108 - 117; 108 - 117 in males and 111 - 116 in females. The ventral scales are wide, about 3 - 4 times the length of a near by dorsal scale, and are rounded. The anal plate is divided and about 1.5 times the length of a ventral.
On the tail the dorsal scale rows at the base of the tail number 11 and are ovate. The subcaudal scales for 18 specimens are divided and number 42 - 56, nine males have 51 - 56, and nine females have 42 - 49. Thus, this species shows sexual dimorphism in the number of subcaudal scales, as does the tail/SVL ratio. The tail is slightly compressed at the base, and becomes more strongly compressed near the distal end. At the base the width is 78% of the height based upon the average of three specimens.
Color and Pattern. Head brown with some mottling. A posterior eye stripe(s) evident in most of the specimens examined. Vertebral row of spots with paravertebral scales involved. A second row of spots occurs on each side on dorsal scale rows eight and nine. Lateral black blotches three scale rows wide are separated by white or yellow bars that are less than one scale wide. This alternating lateral pattern is on scale rows 1 - 6 or 1 - 8. The ventral scales and chin are yellow with some intrusion of black pigment. Two juveniles (BM 1918.104.22.168.1 and LSUMZ 29599) show three rows of dorsal spots; and dark lateral blotches are separated by yellow cross bars about one scale wide. The dark blotches and yellow bars number 32 - 44 each on the body. They continue on to the tail (and number about 22). The dark blotches are on scale rows 1 - 8 or 1 - 6 and some of the dark pigment extends onto the venter. The amount of yellow pigment on the venter is variable. The top of the head has dark spots on an almost black ground color. In adults the three rows of spots found in juveniles become fused into a series of single blotches.
Habitat: Enhydris innominata uses artificial canals, flooded grasslands, and Melaleuca swamp forest (Bryan Stuart, personal communication). This habitat is within the Tonlé Sap-Mekong Peat Swamp Forest ecoregion of Wikramanayake et al. (2002).