224 WATER REPTILES OF TSE PAST AND PRESENT 



advantage. Unlike all other aquatic vertebrates, the turtles never 

 developed real hyperphalangy. Only in the river turtles is there a 

 possibility of an increase in the bones of the fourth digit. 



To discuss in general the structure and habits of the living 

 chelonians would extend this chapter to an undue length, and would 

 add nothing to the many excellent works on natural history 

 accessible to the student. We have therefore contented ourselves 

 with a brief outline of the geological history of the order, with 

 especial reference to their aquatic habits. 



SIDE-NECKED TURTLES. PLEURODIRA 



The suborder of Chelonia, generally known as the snake-necked 

 or side-necked turtles or tortoises, comprises about forty living spe- 

 cies, confined to South America, Africa save the northernmost part, 

 Madagascar, New Guinea, and Australia. In Australia they are 

 the only members of the order known — another instance of the 

 peculiar isolation of the fauna of that region. In the past they 

 lived in North America during Upper Cretaceous times, the earliest 

 known forms of the group in its restricted sense, of which seven 

 species are described by Hay. In Eocene times they are also 

 known from Europe and Asia, from both of which regions they have 

 long since disappeared. 



The Pleurodira, as the term indicates, are easily distinguished 

 from all other turtles by the way in which they withdraw the head 

 within the shell. Instead of withdrawing it by an S-shaped flexure 

 of the neck between the shoulder-blades, as do other turtles, these 

 bend the neck laterally in a horizontal plane, bringing the head 

 within the margins of the shell in front of one or the other foreleg; 

 and the margins of the shell are produced here in an eave-like 

 fashion for the greater protection of the head. In the structure of 

 the shell, which is always fully developed into a box, these turtles 

 do not differ very much from the Cryptodira, though there may be 

 some extra bones in the plastron, as also in the skull. The nasal 

 bones are always, the lacrimals sometimes, well developed; the latter 

 never, the former rarely, found in other groups. The lower jaws 

 articulate a Uttle differently, and the external ear is always fully 

 surrounded by bone. Very characteristic is the bony union of the 



