AIDS TO TEE DIFFERENTIATION OF SNAKES. 345 



The Sub-lingdals. 

 It seems inconsistent that when all other scales worthy of a special name 

 have received scientific titles, the " chin shields " so called, should be exempt- 

 ed, and I always refer to them in my notes as sub-linguals. The number of 

 lower labials that touch the anterior pair is usually recorded, but not so those 

 that touch the posterior pair, though these are as useful as the former. As 

 a rule, two labials touch the posterior pair, but in some Tropkhnotus, three 

 come into contact ; in Bwigarus cceruleus and many Callophis, only one; and in 

 Pla'urus coluhrlnus and Latifasciatus, none at all. 



Lower Labials. 



There are one or two points touching the lower labials which have escaped 

 notice. The suture between the first pair (when such is present) varies in 

 length compared with the suture between the anterior sub-linguals. It is 

 nearly always considerably less, varying from one-third to a half as a rule, 

 but in many of the genus Dipsas it is unusually long, and usually fully 

 equals that between the anterior sub-linguals. The relative size of some of 

 these scales requires speeial mention. One in particular which is usually the 

 largest of the series, and peculiar in having genuate posterior border, is im- 

 portant. In Fordonia and Xenopeltis, &c,, this is the third of the series ; 

 in Bungarus and Callophis, &o., the fourth ; in Psammophis condanarus, 

 and some Drijophis, the fifth ; in many Zamenis, the sixth ; and in Cerberus, 

 and many Tropidoiwtus, the seventh. Again, sometimes no lower labial 

 exhibits this peculiarity, as in Python, and Amhlycephalus. This scale I 

 generally refer to as the "genuate" on account of its posterior border. 



The Eyb. 



Such terms as "small," "moderate," and "large" can at best only convey an 

 uncertain meaning even to those well acquainted with the subject. I record 

 two measurements which are much less indefinite — (1) The horizontal dia- 

 meter which is compared with its distance towards the nostril. In many snakes 

 this diameter about equals the distance to the nostril, as in Bungarus &g. 

 In some it falls far short, as in Coluber oxycephalus, &c., and in some is 

 even as little as a third the distance as in Xenopeltis. In some Dipsas 

 and Ambhjcephalus it is greater than the distance to the nostril. (2) 

 The vertical diameter is compared with its distance to the labial margin. 

 In some snakes, such as Simotes, it about equals the distance ; in others it 

 is less, amounting in Eryv johnii to as little as half or even less ; and again 

 in others it is distinctly greater, amounting in Chrysopelea to about twice 

 the distance. 



I am aware that, owing to the early development of the eye, this organ is 

 relatively larger in the young than in the adult ; hence the above measure- 

 ments must be considered as proximately correct only. 



