72 ON THE PHYSIOGNOMY OF SERPENTS. 



serpents, and this peculiarity is at its niaxinuim in the 

 Acrochordiis, in which the innumerable little compartments 

 of the skin are not at all susceptible of being separated 

 from each other. 



The scales of Ophidians present most usually a smooth 

 unbroken surface ; but in many species they exhibit a 

 logitudinal projection more or less sharp ; sometimes 

 mucronated, sometimes rounded, and occasionally reduced 

 to a sim})le s])lierical protuberance, as in most sea-serpents : 

 these are Avhat are named carinated scales. The keels, 

 sometimes nearly obliterated, as in many of the genus 

 Coluber, only exist in the upj^er ranges of scales near the 

 back. In otlier s})ecies all the ranges are roughened by them; 

 but their development diminishes toward the lower parts, 

 so that the two ranges nearest the abdomen most generally 

 are without them. Several other species of the genus 

 Dipsas, and more especially the Psammophis lacertina, pre- 

 sent, on the other hand, scales scooped out into a longi- 

 tudmal hollow more or less deep ; but the species with this 

 character are very few in number. We do not know the 

 use of these differences in the surface of the scales. It 

 has been supposed that the keels are peculiar to water- 

 snakes ; but it is not so, and we shall afterwards find that 

 species of the same genus, and very nearly allied, differ in 

 little else than the presence or absence of these keels : per- 

 haps they only serve to give a greater firmness to the tegu- 

 ments. 



The mesial line of the lower surface of serpents is 

 generally furnished with scaly plates larger than the rest, 

 but those of the tail are generally of a different form from 

 those on the abdomen. These last are disposed in a single 

 range uniformly prolonged from the anus to beneath the 

 throat, where it disappears. These plates, of which the 

 terminal ones are always divided into tAvo, are sometimes 

 very narrow, as in the Tortrix, Boa, &c., and they have 

 some resemblance to the scales of the trunk. They are 

 more extended in some other serpents ; and in most of those 

 animals they are very broad, mount upwards on the flanks, 

 and surround a considerable part of the circumference of 

 the trunk : then it is that their shape, w holly dependent on 



