246 ZOOLOGY. 
The number of vertebre varies exceedingly. Thus in Pipa there are 
seven, and in Python upwards of four-hundred. The ribs, also,.occur in 
various stages of development. In many of the anourous batrachians, 
they are entirely wanting. They are very numerous in serpents, where. 
however, they are not attached to a sternum. In the Crocodile family, the 
entire thorax is highly developed. In the Chelonia, the ribs and sternal 
plates are so expanded as to form a continuous investment for the body. 
The entire bony structure of the turtles presents considerable deviations 
from the usual type. 
Each class of vertebrate animals possesses a form capable of sustaining 
itself in the air, to a greater or less extent. Thus, among fishes, the 
Pactylopterus and Exrocetus exhibit a power of feeble flight, as a means of 
escape from their rapacious fellows. Flying is the rule, not the exception, 
in birds. The bat, among mammalia, can sustain itself in the air by a true 
process of flight. The Preromys, or flying squirrel, and some other forms, 
can glide through the air for a certain distance by means of the expansion 
of the lateral folds of skin, which are stretched by the agency of the limbs. 
In reptiles, the Draco volans exhibits the same power to the degree possessed 
by the last-mentioned mammal. A lateral fold of skin, supported on several 
ribs, enables this animal to pass to a considerable distance through the air. 
An extinet form of reptile, the Pterodactylus, possessed a power of flight 
much like that of the bat of the present day. The general apparatus is 
similar in both, the principal osteological difference being this, that in the 
reptile but one finger was used to stretch the wings, while in the bat four 
are employed for the purpose. 
The muscles of reptiles are strong, but not well provided with blood, and 
consequently exhibit rather a bleached appearance. They retain their irri- 
tability for a long time after life may reasonably be supposed to be extinct. 
Thus the head of the snapping turtle, Chelonura serpentina, will snap at a 
stick touching it, twenty-four hours and more after decapitation. The 
removal, too, of a great part of the brain, or the severing of the spinal cord, 
is far from producing the same immediately injurious effect as is found to 
supervene in birds and mammalia. 
The brain of reptiles, although superior to that of fishes, is yet consider- 
ably inferior to that of birds. It, however, fills up the cranial cavity to a 
much greater degree than that of the class last described. The surface of 
the brain is smooth, without lobes. The two halves of the cerebrum are 
ovate, and are hollowed out into capacious ventricles. The optic lobes are 
exposed, or not covered by the backward prolongation of the cerebrum. 
The cerebellum is minute, and nearly median. The medulla oblongata is 
large in respect to the rest of the brain, and the nerves have a proportional 
thickness at their exit, exceeding the higher vertebrata in this respect. 
Two modifications of the skeleton are met with among reptiles, a naked 
skin as in the Batrachia, and a series of scales or plates as in the remainder 
of the class. The tessellated epithelium covering the naked skin is con- 
tinually being shed, in patches, or entire, and is generally swallowed by the 
toad and frog. The epithelium of the scaled reptiles is generally shed in 
450 
