EXTINCT ORDERS OF REPTILES. 47 I 



structure of the backbone in fishes ; and as the rocks in which 

 they occur are marine, there can be little doubt but that these 

 Crocodiles were, in the majority of cases at any rate, marine. 

 The most important genera belonging to this order are Teleo- 

 saurus, Steneosaurus, Dakosaurus, Makrospondylus, and Sucho- 

 saurus, the last being from the fresh-water deposits of the 

 Wealden (Cretaceous). 



Sub-order 3. Opisthoccelia. The sub-order of the Opisthocai- 

 lian Crocodiles, including those forms in which the anterior 

 trunk vertebrae are concave behind, is one which can be only 

 provisionally retained. Professor Owen includes in this section 

 the two genera Streptospondylus and Cetiosaurus ; 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 the true position of the 

 reptile to which these belonged. 



CHAPTER LXV. 

 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. ICHTHYOPTERYGIA, 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 amphiccelous, and having 

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

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

 sacrum, and no sternal ribs or sternum, but clavicles were pre- 

 sent as well as an interclavicle (episternum) ; and false ribs 

 were developed in the walls of the abdomen. The skull had 

 enormous orbits separated by a septum, and an elongated 

 snout. The eyeball was protected by a ring of bony plates in 

 the sclerotic. The teeth were not lodged in distinct sockets, 

 but in a common alveolar groove. The fore and hind limbs 



