lyi Introduction. 



two groups of Lorlcata and Sqnamata, the former containing 

 the two orders, Chelonia and Crocodilina ; and the latter the 

 Sauria and the Ojj/ddia. The anterior hmbs are sometimes en- 

 tirely absent, together with the scapular arch ; they are never 

 modified so as to form wings like those of Birds ; the caudal 

 vertebrae very frequently form a series equal in length to the 

 length of the rest of the body; the jaws are usually armed with 

 teeth, which are constantly reproduced during the life of the animal. 

 There are two systemic aortae, which either fuse {Squamata) or 

 anastomose {Lorlcata) with each other in front of the dorsal verte- 

 brae; and in no Heptiles, except Crocodilina, is there a complete 

 separation of the ventricular part of the heart into two cavities. 

 And in Crocodilina, the two systemic aortae arising from the two 

 distinct ventricles, communicate with each other at the base of the 

 heart by the foramen Pauizzae ; so that in all Reptiles the venous 

 and the arterial blood come to be more or less freely intermingled 

 without the interposition of capillaries, at, at least, two points of the 

 vascular system, Copulatory organs are always present, except in 

 Hatteria ; for the peculiarities of which see Dr. Giinther, Phil. 

 Trans. 1867, p. 595. 



The skull is less vaulted and less capacious than in Aves. Traces 

 of the entrance of the two exoccipitals into the formation of the 

 occipital condyle are usually persistent. The os quadratum is 

 sometimes movably, sometimes immovably {Lorlcata and Hatteria^ 

 articulated to the cranial walls, a considerable part of the antero- 

 lateral elements of which remain in the condition of fibro-cartilage, 

 except in OpMdia. 



Amphicoelian vertebrae are found in the existing Geclcotidae 

 and Hatteria, where they are connected, as in Mammals, by inter- 

 central cartilages, in the axis of which are persistent masses of 

 substance representing the cJiorda dorsalis. Vertebrae of similar 

 shape are found in the fossil Enaliosauria and Teleosauria. The 

 Vertebrae are ordinarily proeoelian; and, with the exception of 

 Crocodilina, in which a considerable quantity of intervertebral 

 substance remains between the centra of the vertebrae, they are 

 connected with each other simply by synovial joints. In the 

 Chelonia, the vertebral centra vary very much in shape, especially 

 in the neck and tail, where they may either be amphicoelous, 

 biconvex, or precocious. The neuro-central sutures disappear 



