COLD-BLOODED VERTEBRATA 389 



the hinder end of the skull, of the parietal and squamosal (with which 

 is fused a supratemporal, separate in lizards). The fossse are (a) the 

 orbital fossa (orbit) in front of the postorbital bar, bounded also by 

 frontals, prefrontals, and maxilla; ; (A) the supratemporal fossa, between 

 the cranium and the supratemporal arcade, bounded by the postorbital 

 bar in front and the post-temporal behind ; (c) the infratemporal fossa, 

 similarly placed between the two arcades. 



A similar arrangement is found in crocodiles, where, however, the 

 bones are stouter and the fossae smaller. In lizards, by the loss of the 

 quadratojugal, the infratemporal arcade is abolished. In snakes (and 

 some lizards) the supratemporal arcade and fossa also disappear. 

 Turtles have a single broad arcade, representing the two of the tuatara, 

 and thus should have a supratemporal fossa, but this is often bridged over 

 by plates of bone from the squamosal and parietal which form a vaulted 



Fig. 282. — A section through the skin of a lizard. — From Shipley and 

 MacBride. 



i, " Epitrichial layer " of clear cells ; 2, heavily cornified cells forming the scale ; 3, 

 pigment cell ; 4, ordinary cells of horny layer ; 5, innermost Malpighian layer ; 

 6, dermis. 



outer roof to the skull (Fig. 285). [If we compare the arrangement in a 

 frog, we shall find that the lower arcade is present, but that the absence 

 of the upper arcade and the postorbital bar leaves one large fossa at the 

 side of the cranium. The same is the case in birds. Mammals have 

 an arcade, the zygomatic arch, composed of elements (jugal and squa- 

 mosal) which belong to both the upper and lower arcades of the reptiles. 

 They have therefore the supratemporal fossa.] The result of these 

 dispositions is that in crocodiles, turtles, and the tuatara the upper jaw 

 is held rigid, in lizards it has a good deal more elasticity, and in snakes 

 it is extremely elastic, so that the animal can swallow very large prey. 

 The elasticity of the jaws of snakes is further increased by the absence 

 in them of a process from the exoccipital which in other reptiles struts 

 the quadrate, and, with the post-temporal bar, encloses a space some- 

 times called the post-temporal fossa. 



The heart of reptiles consists of sinus venosus, right and 

 left auricles, and ventricle partly divided by an incomplete 



