Hypnale walli, or Wall's hump-nosed viper,[1] is a venomous pitviper species endemic to Sri Lanka. The smallest member of its genus, it is distinguished by having a strongly upturned nose and lower scale counts. No subspecies are currently recognized.
The specific name, walli, is in honor of Ceylonese-born British herpetologist Frank Wall.[2]
The smallest member of the genus Hypnale, the only male with a complete tail measured 30.5cm (12inches) in total length with a tail of 4.3frac=4NaNfrac=4 (14% of total length), while the largest female was 28.3frac=4NaNfrac=4 in total length with a tail of 3.5frac=4NaNfrac=4 (12% of total length). The body is stoutly built.[1]
This species is distinguished from H. hypnale by a snout that has a strongly upturned tip. This is due to an extended rostral scale, which is immediately followed by a raised wart-like hump covered with 10 minute scales. Also, the hemipenes have clearly visible spines. It is distinguished from H. nepa by its lower scale counts.[1]
The scalation includes 17 rows of dorsal scales at midbody that usually lack keels, 7 supralabial scales, 120–126 ventral scales, and 28–33 subcaudal scales.[1]
Regarding the color pattern, Gloyd and Conant (1990) examined a number of preserved specimens, mentioning that some were so faded as to render the pattern almost invisible. One of these, however, had a faint narrow stripe down the center of its back. In general, the color pattern is described as consisting of a series of 18–24 dorsolateral small subtriangular brown blotches, pointing upwards. These are slightly darker than the ground color, except for the upper edges that may be considerably darker. A pair of dark brown blotches are present on the side of the head, along with a pair of dark stripes curving backward on the sides of the neck. The ventral surface is strongly flecked and dappled with grayish brown.[1]
Hypnale walli is found in Sri Lanka. The type locality given is "Kanneliya Forest, Udugama Southern Province, Ceylon [Sri Lanka], elevation approximately 1000abbr=onNaNabbr=on".
According to Gloyd and Conant (1990), the taxonomic status of this species is unclear. Although described here as a species, it may eventually be classified as a subspecies of H. nepa, or even as the minimum extreme for H. nepa with regard to its low ventral and subcaudal scale counts, as well as its relatively short tail.[1]