EEPTILIA. 



213 



upon detached teeth found in a dolomitic conglomerate near 

 Bristol, which has sometimes been supposed to be of Permian 

 age, but which appears to be undoubtedly referable to the 

 Trias. Teeth having the same generic char- 

 acters have also been brought to light in the 

 Triassic deposits of North America. In some 

 respects the Thecodont Eeptiles make an ap- 

 proach to the Lacertilians, while in others 

 they approximate to the Deinosauria. Upon 

 the whole, however, they would seem to be 

 best regarded as an ancient group of Amphi- 

 ccelian Crocodiles, distinguished by their com- 

 pressed, trenchant, and serrated teeth. 



Lastly, the sub-order of the Ojnstliocmlian Crocodiles, in- 

 cluding those forms in which the anterior trunk vertebrse 

 are concave behind, is one which can be only provisionally 

 retained. Professor Owen includes in this section the two 

 genera Streptospondylus and Cetiosatcrus ; but the latter is 

 referable to the Deinosauria, and will be treated of when 

 that order is considered. The genus Streptospondylus has 

 been founded on vertebrae obtained from the Oolitic and 

 Wealden formations ; but there are doubts as to tlie true 

 position of the Eeptile to which these belonged. 



Fig. 563.— Tooth of 

 Palceosaurus 'platyo- 

 don. Trias. Brit- 

 ain. 



