io8 



REPTILES 



chelonians, such as Thalassemys. The turtles themselves show 

 their special adaptation to a pelagic existence by the heart- 

 shaped form and marked depression of the shell (this being the 

 form best adapted for swimming), and by the conversion of 

 the limbs into complete paddles, which, however, retain some 

 of the claws, and are thus suited also for progression, although 

 of a poor kind, on land. The fore-limbs are much larger than 

 the hind-pair, and form the main instruments in propelling the 



Fig. 7. — A, The Luth or Leathery Turtle, to show the paddle-like limbs 

 and shell covered with leathery skin. B, Portion of bony shell showing mosaic- 

 like structure. 



body through the water. A very similar modification in form 

 is presented by the leathery turtle or luth, in which, however, 

 the shell is composed of a mosaic-like pavement of small bones 

 lying loosely on the ribs, instead of a comparatively small num- 

 ber of larger bones firmly welded to the latter ; the fore-paddle 

 being also relatively longer. There is a difference of opinion 

 among naturalists as to the relationships between the Chelonidce 

 and the Dermochelyidce. By some authorities each group is be- 

 lieved to have been independently evolved from terrestrial or 

 fresh -water tortoises, while others regard the luth and its extinct 

 allies as specialised, and as regards the structure of the shell de- 



