398 REPTILES xv. 3- 



the dorsal regions of the hemispheres to form the cerebral cortex, 

 whereas in the remaining reptile groups, and in the birds, the ventral 

 portion has become large, the dorsal thin. The eyes are probably the 

 chief receptors of chelonians, but the nose is also well developed and 

 the animals are very sensitive to vibration. The tympanum is often 

 covered with ordinary skin and hearing is probably not acute (p. 385). 

 The voice is also small. 



The various species show many special habits, some of them com- 

 plicated and ingenious, especially among the aquatic forms (Fig. 227). 

 For instance, the snapping turtles (Chelydra) and alligator turtles 

 (Macroclemys) of North America and Emys in Europe show con- 

 siderable care and skill in stalking and capturing not only fish but 

 also young ducks and other birds. Similarly the smaller turtles, such 

 as Chrysemys picta, the painted terrapin, with bright yellow, black, 

 and red colours, feed not only on insect larvae, but also on flies, which 

 they catch near the water surface. Many observers have shown that 

 the common Grecian tortoise has a marked sense of locality, returning 

 to a favourite spot even after hibernation. 



Our knowledge of the geological history of the Chelonia extends 

 back to the Trias. *Triassochclys (Fig. 222) was an early turtle, with 

 a shell like that of modern forms, but still possessing teeth on the 

 palate. The skull was anapsid and the pectoral girdle contained inter- 

 clavicles, clavicles, and perhaps cleithra; these dermal bones were 

 already somewhat enlarged and incorporated in the plastron. The 

 head, tail, and limbs could not be withdrawn into the shell and were 

 protected by spines. In the later evolution of the Chelonia retraction 

 of the head became possible by one of two methods. In the suborder 

 Pleurodira, 'side-neck turtles', the neck is folded sideways. The group 

 is known from the Cretaceous and survives today in tropical Africa 

 (Chelys), South America, and Australia. The more successful group 

 is the suborder Cryptodira, in which the neck is curved in a dorso- 

 ventral plane. This type is also known from the Cretaceous and 

 includes most of the modern types. The aquatic chelonians show 

 various modifications and it is probable that several lines have inde- 

 pendently returned to the water. As a result of this habit the bony 

 shell is often reduced, presumably in the interests of lightness and 

 because of absence of enemies. This had occurred already in *Archelon 

 of the Cretaceous, which is very similar to the modern Chelone. 

 Dcrmochclys, the leathery turtle, has a curious 'carapace' consisting 

 only of a mosaic of small bony plates beneath its leathery skin, 

 and Trionyx is a freshwater turtle with a soft shell and no horny 



