366 AN INTRODUCTION TO THE STUDY OF FOSSILS 



uln. . 



Fig. 158. — The marine turtle, Archelon ischyros Wieland, from the Pierre (Upper 

 Cretaceous) of South Dakota. Dorsal view ( X 4V)- The animal was protected 

 (i) dorsally by a carapace consisting of marginal plates, ribs, etc., united and 

 covered by leather or horn ; (2) ventrally by a plastron of dermal bones similarly 

 covered. The jaws have no teeth, but were covered by horn. The distal portion 

 of the right hind limb has been restored ; this was bitten off when the animal 

 was young, the end of the tibia and fibula, where the bite occurred, showing healed 

 surfaces, car. i to 5, carpals i to 5, i.e. the distal row of carpal (wrist) bones; 

 CO., coracoid; e.neu., epineural plates of the carapace; these dermal plates are 

 usually undeveloped in the Chelonia, only the neurals — the expanded processes 

 of the vertebrae — being developed; int., intermedium; il., ilium; mar., the 

 dermal marginal plates of the carapace ; m.car., metacarpals ; m.tar., metatarsals ; 

 ph., phalanges; pi., the dermal pleural (costal) bones, covering only about one- 

 fifth of the length of each rib ; r., ribs (the anterior one of the ten pairs not show- 

 ing) ; ra., radius; rarf., radiale ; /ar., tarsus; //., tibia; m/., ulna; m/;^., the ulnare 

 bone of the carpus; /, //, etc., are the digits, I (thumb or great toe), II, etc., of 

 the hand and foot, which were distinct paddles. (From Wieland.) 



out teeth but are covered with a bird-like horny sheath. 

 (Name < Greek chelone, a tortoise.) 



Here are included the living tortoises and turtles. Undoubted 

 chelonians are known from the Triassic to the present (Fig. 158). 



