192 THE VERTEBRATE SKELETON. 



is remarkably small or even absent. The anterior ribs have 

 two articulating surfaces, and each articulates by its tuber- 

 culum with the transverse process, and by its capitulum with 

 the centrum as in mammals. 



These reptiles occur chiefly in deposits of Triassic and 

 Permian age. Some of the best known genera are Dicynodon, 

 Udenodon, Placodus, Pariasaurus and Galesaurus. They will 

 be noticed in the general account of the skeleton in reptiles. 



Order 2. SAUROPTERYGIA. 



This order includes a number of extinct marine reptiles, 

 devoid of an exoskeleton. The tail is short, the trunk long, 

 and the neck in the most typical forms extremely long. The 

 vertebrae have slightly biconcave, or nearly flat centra. The 

 skull is relatively small and has large supratemporal fossae. 

 The teeth are placed in distinct sockets, and are generally 

 confined to the margins of the jaws ; they are sharp and 

 curved and are coated with grooved enamel. The premaxillae 

 are large, and there is an interparietal foramen. The quadrate 

 is firmly united to the cranium. The anterior nares are 

 separate and are placed somewhat close to the orbits. There 

 is no ossified sclerotic ring. The palatines and pterygoids 

 meet the vomers, and more or less completely close the palate, 

 and in some forms, e.g. Plesiosaurus, there is a distinct para- 

 sphenoid. Thoracic ribs are strongly developed and each 

 articulates with its vertebra by a single head. The cervical 

 vertebrae have well-marked ribs, which articulate only with 

 the centra, in this respect differing from those of Crocodiles. 

 The caudal vertebrae bear both ribs and chevron bones, and 

 abdominal splint-ribs are largely developed. 



In the shoulder-girdle the coracoids are large and meet in a 

 ventral symphysis ; precoracoids and a sternum are apparently 

 absent, but parts generally regarded 1 as the clavicles and 



1 According to Hulke they should be regarded as the omosternum, 

 the clavicles and interclavicle being wanting. 



