JUEASSIC FAUNA. 163 



the animal was viviparous, a supposition in a measure strengthened 

 by the ill-adaptation of its structure to breeding on the land-surface. 

 Not impossibly, however, these animals may have been in the habit 

 of devouring their young, or the young of allied species, as it seems 

 many species of snake do at the present day. 



In the Jurassic rocks we meet with the first traces of that ex- 

 traordinary group of reptiles, the Pterosauria, which, in the pos- 

 session of a tegumentary membrane stretched between the gi-eatly 

 elongated outer digit of the anterior limbs and the bases of the 

 hinder extremities, resembling in many respects the flying-apparatus 

 of bats, were enabled to navigate the air in the manner of birds. 

 To these last, the pterodactyls, as the members of this order are 

 familiarly designated, were closely related in the general conforma- 

 tion of the skull, the pneumaticity of the bones, and the presence 

 of a well-developed keel to the sternum or breast-plate, a character 

 among recent animals found only in birds and bats. But while 

 possessing these and other avian features, the pterodactyls depart 

 in many important particulars from the bird type, and notably in 

 the presence of true teeth implanted in sockets, as in the Croco- 

 dilia, the structure of the manus, the absence of a feathery integu- 

 ment — the animal having been apparently provided with a naked 

 skin — and the possession of a tail composed of distinct vertebrae.* 

 Despite these important differences, however, it may, perhaps, be 

 deemed doubtful whether the animals in question have not as 

 much right to be considered birds as reptiles, the more so as the 

 one great feature separating them from modern birds, the pres- 

 ence of alveolar teeth, has recently been shown to be character- 

 istic of some, if not of most, of the ancient birds. "While, there- 

 fore, it may not be possible to decide upon the exact position oc- 

 cupied by these singular organisms, there can be but little doubt 

 that they, or possibly some closely-allied predecessors with which 

 we are not as yet acquainted, represent the primitive stock whence 

 the type of the modern flying or carinate bird has been evolved. 

 The birds would then have a double line of ancestry, the one here 

 indicated, and another, culminating in the struthious or non-carinate 



* Professor Marsh has shown that at least in some forms of pterodactyls 

 (Ehamphorhynchus) the extremity of the tail was provided with a te.su- 

 mentary expansion, or vertical rudder, by means of which the animal doubt- 

 less guided its flight. 



