PLESIOSAURIA. 395 



They are all placed in one family and in about four to six genera. Con- 

 sidering the great abundance of individuals as indicated by the frequency 

 with which their remains are found, this is a remarkable fact. 



In Mixosaurus Baur, the genus to which all the Triassic remains are 

 assigned, the teeth are more variable in size, and the limbs are less paddle- 

 shaped, the radius and ulna being longer than broad and having a small 

 space between them. Shastasaurus Merr. from the Upper Trias of Cali- 

 fornia may possibly belong to this genus. Ichthyosaurus Konig is the 

 typical and most common genus. It is most common in the Lias but 

 extends into the Cretaceous. Ophthalmosaurus Seeley is edentulous or 

 almost so, and the carpal and metacarpal bones and phalanges are round 

 and not pressed together as in the preceding genera ; Upper Jurassic and 

 Cretaceous of England. Baptanodon Marsh is completely edentulous, pos- 

 sibly belongs to the last named genus, Jurassic of Wyoming. It is quite 

 clear from this small amount of diversity of structure that we are acquainted 

 with but a minute fraction of the group. There must have been, possibly 

 in Triassic and earlier times, an immense number of forms the remains 

 of which have yet to be discovered, and which may, when they are 

 discovered, enable us to relate these isolated creatures to other repti- 

 lian groups. 



Sub-class 7. PLESIOSAURIA.* 



Amphibious or marine lizard-like reptiles with long neck, two pairs of 

 five-toed limbs, and strongly developed pectoral and pelvic girdles. The 

 skull has a supratemporal fossa only, a single broad temporal arcade, large 

 pterygoids which reach the vomers and meet in the middle line, fixed quadrates, 

 and a pineal foramen. Triassic to Cretaceous of Europe, N. and S. America, 

 N. Zealand, and India. 



In the triassic forms the limbs appear to have been less modified than 

 in the post-triassic and to have been adapted for walking. In the post- 

 triassic forms they are paddle-like and provided with an increased number 

 of phalanges though not of digits. The larger species may attain a length 

 of 40 feet. The vertebrae are weakly biconcave or flat, and the neural 

 arches are usually suturally united to the centra. The number of cervical 

 vertebrae is always great (20 to 40) and the tail, especially in the marine 

 forms, is short. All the vertebrae carry ribs except the atlas and the 

 axis and the hinder caudals. The cervical ribs are articulated to an arti- 

 cular surface on the centra alone ; they are two-headed in the earlier 

 genera, single-headed in the later forms. In the trunk the ribs are one- 

 headed and articulated to a transverse process of the neural arch or more 

 rarely directly to the neural arch. There are two to four sacral vertebrae, 

 which are not fused. The caudal vertebrae have chevrons, and their 

 ribs are articulated to the centrum. 



The features of the skull are clearly shown in the diagrams (Fig. 220) 

 illustrating the skull of Plesiosaurus. The premaxillae are large and form 

 the short rostrum. The anterior nares are near the orbit. The orbit is 

 closed behind by the jugal and a bone which corresponds to the post- 

 orbital and postfrontal. There are large supratemporal fossae, and the 

 temporal arcade is formed by the backward continuation of the jugal 

 and of the bone which is supposed to consist of postfrontal and post- 



* Called by O\ven Sauropterygia. 



