Wieland : Plastron ok the Protostegin^e. <) 



Dermochelys amongst existing, and amongst extinct forms, only in the 

 several genera of the Thalassemydidae of the European Jurassic and 

 Cretaceous, together with Protosphargis of the scaly clays of Italy. 



The epiplastron of Archelon as represented in superior view in 

 Fig. i is of subcrescentic outline with the anterior limb heavy, and 

 the posterior broadened, flattened, and digitate. The thickness of the 

 heavy anterior end is 4.5 centimeters, and an accompanying humerus 

 is exactly 2 feet in length. 



As in the Trionychydae there is no true sutural union with the ento- 

 plastron, the contour showing that the superior face of the epiplastron 

 was overlain by the antero-external border of the entoplastron. Be- 

 yond this border the anterior limb of the epiplastron projected about 7 

 cm. like a broad, short, heavy horn, with its convex sideental. Four 



Fig. 2. Aspidonectes spinifer. Nether (ectal) view of plastron. X i « ep, 

 epiplastron ; en, entoplastron ; /, /, anterior and mesial foramina. ( Cf. epiplastron 

 of Archelon and Dermochelys, also of Thalassemydidae.) 



broad, shallow furrows increasing in depth from the inner to the outer 

 side, mark the contact of as many overlying digitations or ridges 

 which may all have been entoplastral, rather than in part hyoplastral. 

 This lack of sutural union and the boomerang-like shape of the epi- 

 plastron show how it must have been the very first bone to be torn out 



