214 G. E. Wieland—On Marine Turtles. 



elements of the normal testudinate carapace. On the other 

 hand, the ribs are more fully developed than those of any other 

 fairly well-known turtle, not even excepting Protosphargis 

 (1, 2) and Dermoehelys. In the latter genus, however, the 

 carapace being replaced, in as far as an outer bony shield is 

 concerned, by a strong osteodermal mosaic, there is no neces- 

 sity for a fully normal development of all the ribs, and such is 

 not present, the anterior pair of ribs being in fact quite as 

 much reduced as in the living Cheloninse (9). In the latter 

 the first rib, after affording support to the distal extremity of 

 the scapula, flattens and ends against the inner surface of the 

 very large first pleural. And this first pleural, which is in the 

 simpler sense only an expansion of the second rib, is in turn 

 strongly united by suture to the nuchal, the whole arrange- 

 ment being one of compactness and great strength. 



In the absence of a strongly joined nuchal and first pair of 

 pleurals, efficient support for the scapular region of the shoulder 

 girdle would be clearly obtained in one of two ways : Either 

 there must be an increased size of the first rib, or additional 

 strength must be secured by dermal thickening, with or with- 

 out the development of osteodermal plates. And it is evident 

 that if one of these conditions is found present it may be 

 justly concluded that the other is absent, or but slightly devel- 

 oped. Hence, because of the great strength of the first rib in 

 Archelon we may conclude that there was no unusual develop- 

 ment of the outer covering of the carapace, whether leathery 

 or of horn, and that there certainly was no osteodermal mosaic 

 as in Dermoehelys. The entire arrangement of the ribs, and of 

 the supporting bones of the shoulder girdle region in Archelon 

 thus forms an exceedingly clear example of compensatory 

 change. There is also involved a return to early and primitive 

 conditions. The Protosteginse (9) as we know were distinctly 

 pelagic, and, accepting the views of Baur and of Dollo, 

 they are, in common with all the marine turtles, descended 

 from ancestors with a normally developed carapace and plas- 

 tron ; while in the case of these ancestral forms we may be 

 quite positive that the neuralia and pleuralia were of strong, 

 and the ribs proper of minor development. Moreover, such 

 forms must, of course, have been descended in turn from 

 Theromeran (3) ancestors in which there was a fully, normal 

 rib development. In Archelon, therefore, the ribs may be 

 considered as having regained a large and full size quite equal 

 to that which must have characterized the early Theromeran 

 ancestry of the Testudinates, although changed fundamentally 

 in position with reference to the shoulder girdle. The neuralia 

 and pleuralia, on the other hand, have undergone much reduc- 

 tion as compared with the marine turtles of to-day, the margi- 



