234 VERTEBRATE ZOOLOGY 



characteristic carapace and plastron, and their horny, toothless jaws, 

 constitute their outstanding characteristics. 



They occupy a very wide range of habitat zones without displajdng 

 any very radical departures from the typical chelonian form and pro- 

 portions. They range from the pure marine types which come on land 

 only for the purpose of laying their eggs in the sand; through a whole 

 series of amphibious forms, living in ponds and spending a consider- 

 able part of the time on land; culminating in the giant purely 

 terrestrial forms that are found on several groups of oceanic islands. 

 Their adaptive radiation does not include arboreal, cursorial or 

 volant types, for the probable reason that the shape and weight 

 of the armature does not readily lend itself to these modes 

 of life. 



Until recently the ancestry of the Chelonia was entirely a mystery, 

 but it is now believed by palaeontologists that the Eunotosauria of 

 Permian times furnish a connecting link between the Chelonia and 

 still more primitive cotylosaurs of the Permo-Carboniferous. The 

 Eunotosauria have been ably discussed by Watson and there is now 

 very general agreement with his contention that these forms repre- 

 sent a group which was ancestral to the Chelonia. 



The order Chelonia is divided into two sub-orders, the Athecae (with- 

 out a true carapace), and the Thecophora (with a carapace). 



Sub-Okder I. Athec^ 



The sole living representative of this sub-order is Dermochelys 

 coriacea, the Leather-back Turtle (Fig. 133, A). Instead of the usual 

 closely-knit carapace and plastron it has twelve longitudinal rows of 

 dermal plates (5 dorsal, 5 ventral and 2 lateral). The homologues of 

 these can be recognized in the scute rows of some of the Thecophora. 

 The limbs are large, flipper-Uke paddles of a highly specialized aquatic 

 type. The tail is rudimentary. Dermochelys has a wide distribution, 

 ranging over all of the inter-tropical seas, but is nowhere abundant. 

 It is carnivorous, feeding chiefly on moUusks, fishes and crustaceans. 

 One of the most peculiar facts about this species is that only large 

 specimens and "babies" have ever been found. Where they pass the 

 many years of their youth and early maturity is a mystery. Possibly 

 there is in some obscure corner of the world an undiscovered Der- 

 mochelys rookery. 



