CHELONIA 



243 



a pygal, probably one or more suprancurals, and about eleven 

 peripherals on each side. To what extent the neurals and the 

 costal plates had become anchylosed to the neural spines and the 

 ribs respectively, it is now impossible to determine. Nor can we 

 say to what extent the various elements of the carapace had become 

 connected with one another. There was a subdcrmal plastron 

 which was composed of at least eleven bones. 



"According to the author's view, as time went on the external, 

 mosaic-like shell disappeared in most turtles, while a more eflicient 

 armor was developed out of the subdcrmal elements. In the ances- 

 tors of Dcrniochelys, however, the dermal armor was retained, while 

 the more deeply seated one disappeared, with the exception of the 

 nuchal bone." 



Such a hypothesis as the foregoing satisfactorily explains the 

 extraordinary mosaic shell of the leather-back, and is perhaps an 

 acceptable explanation of the rather strange fact that the horny 

 shields of turtles do not correspond with the bones below them, 

 as might be expected. Unfortunately this hypothesis lacks 

 sufRcient proof. About the only evidence that is offered in its 

 support is the existence of a row of bones along the middle line in 

 the Cretaceous Toxockelys, and notably in Archelon, both aquatic 

 forms. It is urged that these bones, the epineurals of Wieland, 

 are really the remains of an external layer that persisted in these 

 turtles. However, they might have been new ossifications, such 

 as we know did occur in not a few of the land tortoises later over the 

 tail and limbs. Aside from Proganochelys in the vast interval of 

 time from the Triassic to the Eocene no other evidence of such 

 an external dermal layer has been discovered. The chief argument 

 against such divergent ancestry of the turtles in two chief lines of 

 descent is the fact that in its other structure Dermochelys shows 

 great resemblance to other sea-turtles of the Cretaceous times — 

 so much resemblance that it seems impossible that the ancestors of 

 the leather-back should have paralleled them in almost ever>'thing 

 except the shell. 



On the other hand, those who disagree with this view believe 

 that the modern leather-back is the descendant of such Cretaceous 

 marine turtles as Protostega or Archelon, some of which had lost 



