On the systematics of Trimeresurus labialis Fitzinger in Steindachner, 1867, a pitviper from the Nicobar Islands (India), with revalidation of Trimeresurus mutabilis Stoliczka, 1870 (Squamata, Viperidae, Crotalinae).

The Asian pitviper currently identified as Trimeresurus labialis Fitzinger in Steindachner, 1867 is revised on the basis of morphological data obtained from 37 preserved specimens originating from seven islands of the Nicobar Islands. Multivariate analyses shows that these specimens can be divided into two clusters of populations which differ by a series of constant taxonomically informative morphological characters. The first cluster, which includes the name-bearing types of Trimeresurus labialis Fitzinger in Steindachner, 1867, is present only on Car Nicobar Island. The second cluster, which includes the name-bearing types of Trimeresurus mutabilis Stoliczka, 1870, is distributed on the Central Nicobar Islands. We regard these clusters as distinct species, which are morphologically diagnosable and isolated from each other. As a consequence, Trimeresurus mutabilis, long considered a synonym of T. labialis, is here resurrected to specific level. A lectotype is designated for Trimeresurus mutabilis.