652 



COMPARATIVE ANATOMY 



The reptile most closely resembling extinct forms is thought to be 

 Sphenodon (Fig. 377). It is confined to a few small islands off the coast 

 of New Zealand and is hunted and eaten by the Maoris. It is also 

 called "tuatara" and lives in burrows. Externally it looks like a lizard, 

 though skeleton and viscera are quite unlike those of other living 

 lizards. 



Order Chelonia. (Turtles and Tortoises). 



These have a bony covering and toothless jaws. The covering con- 

 sists of a dorsal or upper portion called a carapace, and a ventral plas- 

 tron. These plates are soft in very young animals. The surface is cov- 

 ered with horny shields which Gadow believes to be phylogenetically 

 older than the underlying bony plates. These latter do not correspond 

 with the former in either number or position. 



There are two sub-orders. 



Sub-Order I. Athecae. 



These are without a true carapace. There is only one living repre- 

 sentative of this type, namely the Leather-back Turtle, known as Der- 



mochelys (sphargis) coriacea. (Fig. 378.) In- 

 stead of the regular carapace there are five 

 dorsal, five ventral, and two lateral dermal 

 plates. The tail is rudimentary and the limbs 

 are larger flipper-like paddles. Only large and 

 very small specimens have ever been found. 

 No one knows where they live between these 

 stages. 



Sub-Order II. Thecophora. 



These are the true turtles (Fig. 379) which 

 are divided into two groupings known as 

 Cryptodira to which group most of the turtles 

 of the Northern Hemisphere belong. The 

 head is retractile and the pelvis is not fused to 

 the shell, while in Division II, are the Pleuro- 

 dira, representing a large group of the South- 

 ern Hemisphere. These do not retract the 

 head but bend it sideways under the shell. The 

 pelvis is fused to the shell. 



Sphargis coriacea 

 (leather-bade turtle) 



Fig. 378. 



The only chelonian without 

 a true carapace. (From Gadow.) 



The more commonly known American Turtles belong to Division 

 Cryptodira. The snapping turtles being members of the family Chely- 

 dridae. The skunk- or musk-turtle is a member of the family Cinoster- 

 nidae, and the common pond tortoises are members of the family Testu- 

 dinidae. The "tortoise-shell" turtle belongs to those commonly called 

 "sea-turtles" and is a member of the family Chelonidae. 



