G. R. Wieland — On Marine Turtles. 



123 



The epiplastra are doubtless of the form shown in figure 12. 

 Referring to my first description, however, Dr. Hay thinks 

 that the element figured must be the right, not the left 

 member ; superposition would therefore not be of the out- 

 turned Trionychoid type that 1 have supposed. Dr. Hay saw 

 this element soon after it was collected, and is consequently in 

 a position to judge ; nevertheless I think he errs and that the 

 explanation of his opposite opinion is the condition he has 

 observed in the entoplastron of P.potens. Moreover, I am 

 not sure that he has correctly determined the hyo- and hypo- 

 plasia in that turtle, for the elements he figures as xiphiplastra 



Fig. 12. 



Figure 12. — Archelon ischyros. Left epiplastron, x £. Ectal view on 

 the right below and ental view on the left. On the right above, the ante- 

 rior, and on the left, the posterior edge views of the recovered portion are 

 shown. (There is no doubt that the restoration of the thin dactylate end is 

 fairly accurate both as to form and size.) This element was not present in 

 the original type, having only been observed once in all the history of the 

 Protostegidas. 



I should certainly have called hyoplastra. In either case, 

 however, P. potens, the type of which Dr. Hay was kind 

 enough to show us, is a -quite different turtle from any of the 

 foregoing, and the evidence it affords as to the form of the 

 epiplastra is only negative and quite uncertain. It seems 

 much better to accept the positive evidence at hand, which is 

 to the eifect that if the element figured is the true epiplastron, 

 it projected beyond the anterior border of the entoplastron and 

 was borne on it quite as in Trionychids. But rather than risk 

 finality in error, it has not been given a place in the restored 

 type of A. ischyros. 



