198 THE VERTEBRATE SKELETON. 



The sacrum is composed of two vertebrae. Ossified inter- 

 centra (interdorsalia) generally occur in the cervical and 

 caudal regions, and sometimes throughout the whole vertebral 

 column. In the skull the quadrate is immovably fixed and 

 united to the pterygoid. The palate is well ossified, while 

 the premaxillae which are often beak-like are never ankylosed 

 together. The jaws may be toothless or may be provided 

 with teeth which are usually acrodont (see p. 199). The 

 palatines frequently bear teeth, and in Proterosaurus teeth 

 occur also on the pterygoids and vomers. The rami of the 

 mandible are united by ligament at the symphysis except in 

 the Rhynchosauridae, in which the union is bony. Superior 

 and inferior temporal arcades occur. 



The ribs have capitula and tubercula, and often uncinate 

 processes (see p. 190) as in birds. A pectoral girdle and 

 sternum, with clavicles and a T-shaped interclavicle are de- 

 veloped, and abdominal ribs are always found. The precoracoid 

 is however absent. The limbs are pentedactylate. 



Sphenodon 1 (Hatteria) now living in some of the islands 

 of the New Zealand group, is certainly the most generalised 

 of all living reptiles. Though lizard-like in form it differs 

 from all living lizards in the possession of two temporal 

 arcades, abdominal ribs and a fixed quadrate ; and is often 

 considered to be nearly allied in many respects to the type 

 of reptile from which all the others took their origin. 



Among the better known extinct forms are Proterosaurus 

 of Permian and Hyperodapedon of Triassic age. 



Order 6. SQUAMATA. 



This order includes the extinct Mosasaurians, and the 

 lizards and snakes which form the vast majority of living 

 reptiles. The trunk may be moderately elongated and provided 



1 A. Giinther, On the Anatomy of Hatteria, Phil. Trans, vol. 157, 1867, 

 p. 595. 



