204 



FOSSIL TURTLES OF NORTH AMERICA. 



greatest thickness of the entoplastron, at the midline, is 60 mm.; at the narrowest part of the 

 lateral portion, 50 mm. The epiplastra probably overlapt the entoplastral wings nearly to the 

 midline and ran backward a short distance on the borders of the hyoplastra. Wieland (Ann. 

 Carnegie Mus. IV, p. 8) describes what he regards as the epiplastron. For the present writer's 

 opinion on this, the reader is referred to page 190. 



The hyoplastra (fig. 262, hyo) have each an antero-posterior length of 1030 mm.; while the 

 width, inclusive of the spines, is 1 100 mm. The greatest thickness is 48 mm. The articulation 

 with the hypoplastron is about 150 mm. wide. 



The hypoplastron (fig. 262, hypo) from the hvoplastral articulation to the extremities of 

 the processes embracing the xiphiplastron, has a length of about 960 mm. The greatest 

 breadth, including the spines, is 1000 mm.; the greatest thickness is 41 mm. 



Figs. 261 and 262. Archelon ischyros. X '. Nuchal bone and plastron of type. 

 261. Nuchal bone. 262. Plastron, etit, entoplastron; hyo, hvoplastron; hxpo, hypoplastron; xiph, xiphiplastron. 



The xiphiplastra (fig. 262, xiph) are each 500 mm. long, 170 mm. wide, and 36 mm. thick. 

 The proximal end of each fits in between processes of the corresponding hypoplastron, being 

 grooved to receive the borders of those processes. The distal ends of the two xiphiplastra join 

 at the midline by interlocking digitations. These bones are less abruptly curved than are 

 those of Protostega, and greatly resemble those of Protostega advena. 



The total length of the plastron, according to Wieland's figure, is about 2100 mm., being 

 thus longer than the estimated length of the carapace. The latter was certainly the longer. 



The skull (fig. 263) of this species reacht an enormous size both relatively to the length of 

 the carapace and absolutely. Wieland's restoration of the one studied by him is here repro- 

 duced. The same individual furnisht also a nearly complete skeleton. It was about three- 

 fourths the size of the type specimen. The skull has a length of 720 mm. Another, but 

 fragmentary specimen, indicated a skull fully a meter in length. 



