444 MANUAL OF ZOOLOGY sect. 



A series of ossifications — the abdominal ribs, with a mesial 

 abdominal sternum — lie in the wall of the abdomen in the 

 Crocodilia (Fig. 264, Sta), and similar ossifications occur 

 also in the monitors and in Hatteria. The elements of 

 the plastron of the Chelonia are probably of a similar 

 character. 



In the skull ossification is much more complete than in 

 the Amphibia, the primary chondrocranium persisting to a 

 considerable extent only in some lizards and in Hatteria, 

 and the number of bones is much greater. 



The parasphenoid is reduced, and its place is taken by 

 large basi-occipital, basi-sphenoid, and pre-sphenoid bones. 

 The lower jaw articulates with the skull through the interme- 

 diation of a quadrate bone, which is movable in the lizards 

 and snakes, fixed in Hatteria, the Chelonia, and Crocodilia. 



A remarkable feature of the skull of the snakes (Fig. 267), 

 is the free articulations of the bones of the jaws, permitting 

 of the mouth being opened very wide so as to allow the 

 passage of the relatively large animals which the snake 

 swallows whole ; this wide opening of the mouth is further 

 aided by the two halves of the mandible not being firmly 

 fixed together anteriorly, but merely connected together by 

 means of elastic tissue, so that they are capable of being 

 widely separated from one another. 



In accordance with their purely aerial mode of respira- 

 tion, the visceral arches are much more reduced in the 

 Reptilia than in the Amphibia in general. The only well- 

 developed post mandibular arch is the hyoid, and even 

 this may undergo considerable reduction (Ophidia). The 

 branchial arches, except in so far as they may contribute to 

 the formation of the tracheal rings, are not represented in 

 the adult, with the exception of most Chelonia, in which the 

 first branchial arch persists. 



