528 



FOSSIL TURTLES OF NORTH AMKRICA. 



The plastron (plate 106, fig. i) has a breadth, from the extremity of the hyoplastral 

 processes of one side to those of the other, of about 310 mm. The outer fourth of the hyoplas- 



tron of each side has coalesct with 

 the corresponding hypoplastron. The 

 transverse extent of these bones, 

 along this suture, is 133 mm. Where 

 narrowest the bridges are 48 mm. 

 wide. The hyoplastron and hypo- 

 plastron have a close resemblance 

 to those of Platypeltis ferox. The 

 hypoplastron differs from that of the 

 species just named in havingagreater 

 number of digitations directed toward 

 the midline. 



The xiphiplastrals are concave 

 along the outer border, whereas those 

 of P. ferox are convex. In the latter 

 species there is a single process which 

 meets the single process from its fel- 

 low bone; in A. mira there is on the 

 left side a broad process which is 

 received between two slenderer pro- 

 cesses of the right side. 



Nearly the whole inferior surface 

 of the hyoplastra, hypoplastra, and 

 the xiphiplastra is covered with a 

 sculptured layer of bone, the callos- 

 ities, as they are called in living tri- 

 onychid turtles. The pits and ridges 

 are less strongly developt than on the 

 carapace. They are most distinct on 

 the outer ends of the hyoplastra and hypoplastra. Here there are about 5 pits in a 10 mm. 

 line. On the mesial halves of these bones the pits are very shallow. The sculpture on the 

 xiphiplastra is feebly exprest. 



The skull (fig. 684-686) is beautifully preserved. It lacks the lower jaw, the occipital 

 condyle, the supraoccipital process and parts of the squamosal processes. 



The approximate length of the skull, from the tip of the snout to the occipital condyle, 

 is 72 mm.; the greatest width, at the upper posterior border of the tympanic cavity, is 45 mm. 

 From the fronts of these cavities the sides of the skull converge to near the narrow and rounded 

 snout. The flexure of the skull behind the choanae is remarkable, fully as great as in 

 A. cartilaginea, while the descent of the face is still more rapid than in the living species 

 just named. 



The sutures between the bones are yet open, but no remarkable relations have thereby 

 been shown. The tympanic chambers are large, each diameter being 16 mm. The zygomatic 

 fossae are small, only 14 mm. long. The orbits are unusually large, the horizontal diameter 

 being 16 mm. equal to that of specimen of Platypelti s ferox whose skull is 94 mm. long. The 

 vertical diameter is 13 mm. The interorbital space is only 4 mm. wide. Each diameter of the 

 nares is 10 mm. The snout projects 20 mm. in front of the orbits. 



The pterygoid portion of the palate has a width of 25 mm. The anterior portion of the 

 skull is remarkable for the depth of the groove in which the choanae are situated. This begins 

 at the posterior palatine foramina and extends forward to the premaxillae. It has a depth of 5 

 mm. and a width of 8 mm. The choanae are placed well backward, the middle of the length of 

 each being opposite the middle of the orbits. Each is 10 mm. long. The triturating surfaces 

 of the upper jaws are 6 mm. wide opposite the choanae, but they narrow gradually forward. 

 Altho the lower jaw is missing one of the ceratohyals is preserved. Most of the internal 

 skeleton is preserved, but it has not been cleared from the matrix. 



FIGS. 684-686. Amyda mira. Skull of type. Xo.g. 

 684. Seen from above. 685. Lower aspect. 686. Right side. 



