16 ANATOMY OF VERTEBRATES. 



the orbits ; orbits large ; a circle of sclerotic plates. Skin naked, 

 forming a vertical tail-fin (inferential). 



Genus Ichthyosaurus. 



Order XVI. SAUROPTERYGIA. {Extinct) 



Body, in most, with a long neck ; limbs natatory, with not 

 more than five digits ; an episternum and clavicles ; vertebrae 

 with flattened, or slightly cupped, articular surfaces ; a sacrum 

 of one or two vertebrae for the attachment of the pelvic arch, 

 in some ; ribs with simple heads ; no post-orbital and supra- 

 temporal bones ; large temporal and other vacuities between 

 certain cranial bones ; a foramen parietale ; two antorbital nostrils ; 

 teeth simple, in distinct sockets of premaxillary, maxillary, 

 and premandibular bones, rarely on the palatine or pterygoid 

 bones ; maxillaries larger than premaxillaries. 



Genera Plesiosaurus, Pliosaurus, Nuthosaurus, Placodus. 



Order XVII. ANOMODONTIA. {Extinct) 



Teeth wanting, or limited to a single maxillary pair, having the 

 form and proportions of tusks ; a ' foramen parietale ;.' two ex- 

 ternal nostrils ; tympanic pedicle fixed ; vertebrae biconcave ; an- 

 terior trunk-ribs with a bifurcate head, ischiopubic symphysis 

 continuous. 



Fam. Dicynodontia. 



A long ever-growing tusk in each maxillary bone ; pre-maxil- 

 laries connate, forming with the lower jaw a beak-shaped mouth, 

 probably sheathed with horn. Sacrum of more than two verte- 

 bra?. Limbs ambulatory. Ex. Dicynodon. 



Fam. Cryptodontia. 



Upper as well as lower jaw edentulous ; premaxillaries distinct 

 and produced. Ex. Rhynchosaurus. 



Premaxillaries confluent, short. Ex. Oudenodon. 



Order XVIII. CHELONIA. 



Trunk-ribs broad, flat, suturally united, forming with their 



