332 G. R. Wieland—On Marine Turtles. 



Whether or not the keels of the marginalia mark the fusion 

 of a lateral series of elements, corresponding to the epi-neurals, 

 is likewise only conjectural, although it appears that some 

 light may be shed on the subject by Proganoehelys. There is 

 however some uncertainty as to the number of marginals and 

 true significance of the peculiar marginal fringe of spines in 

 this singularly interesting turtle as so carefully studied by 

 Fraas (4) from material recovered under conditions unfavor- 

 able to the exact preservation of structural details. But it is 

 also a most interesting and suggestive fact that small ossicles 

 are irregularly interpolated between the lateral marginals of 

 the Cretaceous Tytoloma, as small triangular elements about 

 l*5 cm on each side. Such are shown at E, E, E in the accompany- 

 ing figure 5. As these epi-marginal ossicles are not equally pre- 

 sent on both the right and left marginals even in the same 

 individual and certainly not always present in all specimens of 

 Lytoloma angusta, they would at first sight appear to be of 

 much less significance, taken by themselves, than are the epi- 

 neurals of ToxocJielys. Nevertheless it would now seem that 

 they do represent a disappearing series that may once have 

 invested the entire margin of the carapace. If so, they form 

 one of the most impressive examples of the very last vestiges 

 of a vanishing series. 



The truth of this hypothesis yet remains to be mainly deter- 

 mined by fossil evidence, which we may hope ere long to dis- 

 cover, if correct. At any rate it is extremely interesting and 

 suggestive to find further traces of an additional osteodermal 

 series in Lyotoloma, whatever may be the homology to that of 

 Dermochelys. 



What the characteristic number of elements in this system 

 as developed in pre-Cretaceous testudinates was, no one has 

 yet attempted to suggest. Nor is it possible to reach a safe 

 conclusion in the absence of further paleontologic evidence. 

 It would appear however that the series was once at least as 

 complex as is the horn-shield and the bony plate series, and 

 that it had some form of alternate or imbricate relationship to 

 both these latter systems. Also, if the osteodermal mosaic of 

 Dermochelys arose from such an additional dermal series, such 

 origin must therefore have been in part by a subdivision pro- 

 cess such as was suggested to Baur by the abnormal breaking 

 up into smaller ossicles along the edges of the pleurals observed 

 by him in Eretmoclielys. Such a subdivision would of course 

 follow the lines of the original system, and could thus very 

 well produce the carapacial carina seen in Dermoelielys. 



It should be especially noted in this connection that such an 

 hypothesis for the more primitive origin of the osteodermal 

 mosaic does not necessarily imply a more ancient origin for 



