AIDS TO THE DIFFERENTIATION OF SNAKES. 341 



is usually so in the genus Simotes. In other cases the internasal suture 

 is decidedly larger than the nasal, constituting a generic distinction iu Nain. 

 The first labial suture is usually the smallest ; however, in some P^ammojyhis 

 and Tropidonotus it is larger than the internasals, and in the genus Plaiurus 

 is the largest of the six sutures, 



Internasals. 



It is more precise to compare the suture between the two fellows in the 

 median line with the suture each forms with the preefrontal than to com- 

 pare the length of these scales with their breadth, for, laterally, they are 

 reflected on to the face to a variable degree, and, again, as the snout narrows 

 anteriorly, the breadth is not consistent throughouf. In the same way the 

 suture between the prsefrontal fellows should be compared with the suture 

 3ach makes with the frontal. 



The variations in the sutures arising between these four scales are of some 

 value in differentiation. As a rule, the length of the suture between the two 

 internasals ia less than tlie suture each makes with the pragfrontal, and the 

 length of the suture between the prgefrontals is greater than the suture each 

 makes with the frontal, as is seen in many Callophis, Lycodon, Naia, 

 &c. In Slmoies and Amhlyceplialus, however, the suture between the inter- 

 nasals is about half that made with the prefrontals ; and the suture 

 between the prgefroutals, half that made with the frontal. In Tropido- 

 notus, as a rule, the suture between the internasals is sub-equal to that made 

 with the praefronta's, and the suture between the praefrontals sub-equal to 

 that made with the frontal. One instance may be mentioned where these 

 measurements will separate species. In DryopMs mycterizans the suture 

 between the inteinasal is about twice that made with the praefrontals, 

 whereas in Dryophis prasinus these sutures are about equal. In both the 

 suture between the prasfrontals is about twice that made with the frontal. 

 My notes in other cases seem to point to similar differences in some species, 

 but one requires so many of a kind before laying down a rule that I hesitate 

 before giving other examples. 



Again, with regard to the praefrontal, its relations on its anterior, external 

 and posterior aspects are of such variability and importance in different 

 genera and species that it is the most important scale on the head for 

 diagnostic purposes. 



Anteriorly when internasal scales are present they are always in 

 contact with the prasfrontals, but when absent, this relationship may be sub- 

 stituted by the rostral, as in Calamaria, or more commonly the nasals, as 

 in the family Hydrophiinoe (except the genus Plaiurus), &c. 



Externally (1) the nasals meet the praefrontals in the vast majority of 

 species, but there are exceptions, (a) In Naia tripudians and Hypsirhina 

 enhydris a departure from the above is brought about by a meoting be- 

 tween the prseocular and internasals, and is a useful feature in separating 

 these species from their allies. In other instances this peculiarity is incon- 

 stant and valueless, as in Xylophh perroteti^ Ceriems and Hypsirhina sieboldtt, 

 19 



