246 ZOOLOGY. 



The number of vertebrae 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 hatrachians, 

 they are entirely wanting. They are very numerous in serpents, Vv'here, 

 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 

 ])lates are so expanded as to form a continuous investment for the body. 

 The entire bony structure of the turtles presents considerable deviations 

 from the u>:ual type. 



Each class of vertebrate animals possesses a form capable of sustaining 

 Jtseli' in the air, to a greater or less extent. Thus, among fishes, the 

 Bactyloptcrus and Exoccetus 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 Pteromys, or flying squirrel, and some other forms, 

 can glide through the air for a certain distance by means of the expansion 

 ot 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 

 l)y 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 extinct form of reptile, ihe 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 

 leptile 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 trom 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 

 i>vate, 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 



