INTEGUMENT 15 



ning in longitudinal series parallel with the axis of 

 the body, as is the rule, they are sometimes disposed 

 obliquely, and among those in which we meet with 

 this peculiarity several genera are further remarkable 

 in having some of the oblique lateral scales furnished 

 with a serrated keel, to which we shall again allude 

 in the chapter on Habits, when dealing with the 

 rustling sounds produced by certain snakes. The 

 number of longitudinal series of scales on the body 

 varies from 10 (Herpetodryas) to nearly 100 {Python, 

 Boa) ; in the European species from 17 (Contia 

 modesta) to 50 {Eryx jacuhis). The scales are some- 

 times furnished near the end with one or two shallow 

 impressions, termed "apical pits," which afford indica- 

 tions for the distinction of genera and species ; unless 

 of a lighter or darker colour, as is often the case, 

 these pits are not always easy to see, except in a 

 strong light and with the aid of a powerful magni- 

 fying glass. 



The ventral shields, also called " gastrosteges," 

 usually occupy the whole width of the belly ; but 

 they may be much narrower — in Eryx, for instance. 

 They are sometimes bent at an angle on the sides, and 

 this angle may even form a sharp keel, accompanied 

 by a notch in the posterior border, corresponding to 

 the keel, as in several of the more arboreal genera of 

 Colubrids. The shields under the tail, termed sub- 

 caudals or "urosteges," are sometimes similar to the 

 ventrals, but more often disposed in pairs ; in certain 



