72 
ON THE PHYSIOGNOMY OF SEEPENTS. 
serpents, and this peculiarity is at its maximum in the 
Acrochordus, 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 simple spherical protuberance, as in most sea-serpents : 
these are what are named carinated scales. The keels, 
sometimes nearly obliterated, as in many of the genus 
Coluber, only exist in the upper ranges of scales near the 
back. In other species 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- 
tudinal 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 two, are sometimes 
very narrow, as in the Tprtrix, 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, wholly dependent on 
