368 REPTILIA. 



"Thecodont" Reptiles are defined as follows: "Vertebral 

 bodies biconcave ; ribs of the trunk long and bent, the anterior 

 ones with a bifurcate head ; sacrum of three vertebrae ; limbs 

 ambulatory, femur with a third trochanter. Teeth with the 

 crown more or less compressed, pointed, with trenchant and 

 finely serrate margins, implanted in distinct sockets." (Owen.) 



The Thecodont Reptiles are Triassic, and the three most im- 

 portant genera are Thecodontosaurus, Palaosaurus, and Belodon, 

 the last from undoubted Triassic strata, whilst the two former 

 occur in a dolomitic conglomerate near Bristol, which has 

 sometimes been thought to be Permian, but which is also 

 almost certainly Triassic. In some respects these reptiles 

 make an approach to the Lacertians ; but, on the whole, little 

 doubt can be entertained as to their truly belonging to the 

 Amphicoelian Crocodiles. 



The sub-order of the Opisthocxlian Crocodiles, including 

 those forms in which the anterior trunk vertebrae are concave 

 behind, is one which can be only provisionally retained. Pro- 

 fessor Owen includes in this section the two genera Strepto- 

 spondylus and Cetiosaurus; but the latter is referable to the 

 Deinosauria, and will be treated of when that order is con- 

 sidered. The genus Streptospondylus has been founded on 

 vertebrae obtained from the Oolitic and Wealden formations ; 

 but there are doubts as to the true position of the reptile to 

 which these belonged. 



CHAPTER XXXIII. 

 EXTINCT ORDERS OF REPTILES. 



IT remains now to consider briefly the leading characters of 

 five wholly extinct orders of Reptiles, the peculiarities of which 

 are very extraordinary, and are such as are exhibited by no 

 living forms. 



ORDER V. ICHTHYOPTERYCJIA, Owen ( = Ichthyosauria, Hux- 

 ley). The gigantic Saurians forming this order were distin- 

 guished by the following characters : 



The body was fish-like, without any distinct neck, and pro- 

 bably covered with a smooth or wrinkled skin, no horny or 

 bony exoskeleton having been ever discovered. The vertebrae 

 were numerous, deeply biconcave or amphicoelous, and having 

 the neural arches united to the centra by a distinct suture. 

 The anterior trunk-ribs possess bifurcate heads. There is no 



