22 REPTILES 



development of ball-and-socket (instead of nearly flat or slightly 

 hollowed) articulations to the vertebrae ; the socket being in front 

 and the ball behind, thus constituting the proccelous type of 

 vertebral structure. 



From a branch arising directly from the rhynchocephalian 

 stock, without the intervention of the belodonts, may probably 

 be derived plesiosaurs (Sauropterygia) and tortoises and turtles 

 (Chelonia) ; these two orders having many points in common, 

 such as a single (upper) temporal arcade, a fixed quadrate-bone, 

 the mode of articulation of the ribs to the vertebral column, and 

 the presence of clavicles, and of either a true plastron or a system 

 of abdominal ribs. Specialisation has, however, taken different 

 lines in the two groups : in the one case the protection of 

 the body in a shell, together with the loss of the teeth ; while 

 in the other socketed teeth are retained, and the limbs in the 

 more specialised forms become modified into paddles. Although 

 the plesiosaurs can be traced downwards into terrestrial or fresh- 

 water forms, we are at present quite ignorant of the ancestry of 

 the chelonians. 



The placodonts, or bean-toothed reptiles (Placodontia) of 

 the Trias, were long regarded as members of the theromorphous, 

 or mammal-like brigade, but a later view is to consider them as 

 members of the branch which gave rise to the chelonians and 

 plesiosaurs, with both of which they agree in the general 

 characters of the skull, while they resemble the latter in their 

 socketed teeth. A peculiar feature, as will be noticed later, is 

 the extension of the teeth on to the palate, and their crushing 

 type of crown. 



A totally independent branch from the main stem is formed 

 by the pterodactyles, or flying reptiles (Ornithosauria), the whole 

 of whose organisation is profoundly modified for the purpose of 

 aerial flight. Pterodactyles retain evidence of affinity with the 

 ancient parasuchian (and thence with the rhynchocephalian) 

 stock in the retention (except in the more specialised forms) of 

 socketed teeth, fixed quadrates, double temporal arcades, and 

 distinct clavicles. Although the group cannot at present be 

 traced into direct connection with terrestrial, non-volant reptiles, 

 it is quite clear that it has nothing to do with birds. 



The branch which has given rise to the largest of all reptiles 

 is that of the Dinosauria, which, in the opinion of some author- 



