EXTINCT ORDERS OF REPTILES. 445 



Sub-order 2. Amphic(elia.-^'V!\& Amphicoelian Crocodiles, 

 with biconcave vertebrse, are entirely extinct. They have but 

 a limited geological range, extending only from the Lias to 

 the Chalk inclusive, and being therefore strictly Mesozoic* 

 The biconcave vertebrae show a decided approach to the 

 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 Telco- 

 saurus, Sieneosaurus, Dakosaurus, Makrospondylus, and Sttcho- 

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

 Wealden (Cretaceous). 



Sub-order 3. Opist/toccelia. — This sub-order, like the last, is 

 entirely extinct, and is exclusively Mesozoic, all the known 

 examples occurring in the Liassic, Oolitic, and Cretaceous 

 rocks. The most important genera are Streptospondylus and 

 Cetiosaurus. The Cetiosaurus longus of the Upper Oolites 

 (Portland Stone) must have been the largest of all known Cro- 

 codilia, the vertebrae of the tail measuring as much as seven 

 inches in length, and more than seven inches across. 



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 homy or 

 bony exoskeleton having been ever discovered. The vertebrae 

 were numerous, deeply biconcave or ampliicoelous, and having 

 the neural arches united to the centra 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- 



* If the so-called " Thecodont " Reptiles, such as T^ecodontosaurus 

 and Belodon, belong to this sub-order, then the Amphiccelian Crocodilia 

 date from the age of the Triassic rocks. 



