Wieland : Plastron of the Protostegin^e. 



11 



carapace and similar plastral type, which might stand in an ancestral 

 or approximately ancestral relationship. Nevertheless, Dermochelys 

 has retained its isolated position ; fossil evidence bearing directly on its 

 origin has been singularly lacking, in fact going scarcely further than 

 to indicate that Psephof>homs of the Belgian Pliocene may have 

 marked the culmination in 

 size of the'Dermochelydidae. 

 For a time, however, after 

 Cope's description of Protos- 

 tega in 1875 this genus, as 

 very imperfectly known, was 

 supposed to largely bridge the 

 gap between Dermochelys and 

 the other marine turtles, 

 mainly on the ground of its 

 considerable carapacial and 

 its doubtless complete horn- 

 shield reduction. The dis- Fig. 4. Ental view of nuchal, of (a) Aspi- 

 covery, in much better preser- danectes spinifer ( X I), and (*) the entoplas- 

 " , tl _ , , i.i tron of Archelon ischyros (X fa)> *> sma11 



vation, of the closely related nether tubercularprocessarticulatingwithneU ral 

 Archelon, as well as the Study arch of cervical ver tebra; r, a lateral ridge for 

 of better specimens of Proto- muscular attachment. Entoplastral and nuchal 

 Stega, however, developed the similarity are correlated in these forms, 

 presence of far closer relation- 



ship to the Cheloniidee than was at first suspected, the writer finally 

 being led to include these forms in a Chelonidan subfamily, this 

 doubtless being their correct morphologic rather than their exact 

 phyletic position. 



Nevertheless it now becomes possible to coordinate several hitherto 

 isolated facts. If we regard Dermochelys as the most specialized 

 Testudinate, and the osteodermal mosaic as a secondary structure, the 

 plastron has been more persistent than the carapace, only the ento- 

 plastron having been lost by reduction, whereas the nuchal is the sole 

 remaining carapacial element. The Protosteginse also, though struc- 

 turally speaking members of the Cheloniidae, are now seen to have 

 with their much reduced carapace the same highly characteristic epi- 

 plastral type as Dermochelys, as well as other minor resemblances 

 which need not now be enumerated. The same is true of the Thalas- 

 semydidae of the European Jurassic and Cretaceous, as represented by 



