CHELONIA 



235 



farther on, is really the descendant of these or allied turtles, as many 

 authors believe, it of course represents the very highest aquatic 

 specialization of all Chelonians. If, on the other hand, as some 

 believe, the leather-back is the end of a long and independent line 

 of descent, then Archelon represents the highest aquatic specializa- 

 tion of all other turtles. 



In size, at least, Archelon attained the maximum of the order, 

 reaching a length of more than twelve feet, and a weight of more 



Fig. 124. — Archelon from below, without plastron: //, humerus; ;-, radius; u, ulna; 

 sc, scapula; c, coracoid; p, pubis; i, ischium. (From Wieland.) 



than three tons. Except that the shell was not heart-shaped or 

 elongated as in all modern sea-turtles, but nearly circular in out- 

 line, it had all the aquatic adaptations of the sea-turtle in a yet 

 higher degree. 



The shell was depressed; the dermal plates covering the ribs 

 had almost entirely disappeared, remnants only of their upper 

 ends remaining; the skull (Fig. 127) had the temporal region 

 wholly roofed over; the neck was short and not retractile. The 



