392 REPTILES xv. 2- 



through them on to the outer surface of the skull, an arrangement 

 that increases their mechanical advantage. 



In many reptiles, two such fossae appeared, the condition being 

 termed diapsid. This is seen in the subclasses Lepidosauria and Archo- 

 sauria, perhaps the most successful groups of reptiles. In lepidosaurs 

 of the order Squamata, however, the lower temporal arch is always 

 incomplete, having no quadrato-jugal bone and the jugal separated 

 from the squamosal. In some lizards and in snakes the upper arch 

 is also lost. 



In other groups only a single fossa and arch is present. When this 

 is situated high on the skull the condition is known as parapsid. 

 Parapsid skulls are seen in the subclasses Ichthyopterygia (icthyo- 

 saurs) and Synaptosauria (plesiosaurs, &c). Formerly, these two sub- 

 classes were placed together in a group known as the Parapsida, as is 

 shown in Fig. 221, but this classification is now regarded as artificial, 

 since the ichthyosaurs and sauropterygians are not closely related; 

 in fact a careful analysis shows that the bony relationships of their 

 single temporal fossae were rather different. 



In the remaining subclass, the Synapsida, there is also a single 

 fossa, but in the earlier forms at least it is placed low down, and is 

 bounded below by the jugal and squamosal. The term synapsid, 

 meaning 'fused arch', is actually a misnomer, due to the fact that 

 early workers believed, wrongly, that the single arch was formed from 

 the fusion of the two seen in diapsids. 



The synapsids comprise the mammal-like reptiles, but in the later 

 members of the group, such as *Cynognathas, and in their descendants 

 the mammals, the temporal fossa has greatly enlarged, and has lost 

 its primitive relationships. 



3. Order 1. Chelonia 



Shut away in their boxes the tortoises and turtles have retained 

 some of the features of the earliest anapsid reptiles. Even today they 

 are a not unsuccessful and quite varied and widespread group, with 

 more than 200 species. These include terrestrial animals, such as 

 Testudo graeca, the Grecian tortoise of south Europe, which is her- 

 bivorous; the freshwater tortoises, such as Chrysemys and other 

 American terrapins, and Emys the European water-tortoise, all of 

 which are carnivorous. The marine Chelonia, usually known as 

 turtles, are often very large. Dermochelys, the leathery turtle, which 

 has no horny shell, is over 6 ft long and weighs half a ton. Chelone 

 my das, the green or edible turtle, is over 3 ft long. 



