528 FOSSIL TURTLES OF NORTH AMERICA. 
The plastron (plate 106, fg. 1) 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 Platy peltis jerox. The 
hypoplastron differs from that of the 
species just named in havinga greater 
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 4. 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- 
Fics. 684-686.—Amyda mira. Skull of type. Xo0.9. 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, 1s 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 choane 1s 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 
foss are small, only 14 mm. long. The orbits are unusually large, the horizontal diameter 
being 16 mm. equal to that of specimen of Platy peltis 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 10mm. 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 choanz are situated. This begins 
at the posterior palatine foramina and extends forward to the premaxilla. It has a depth of 5 
mm. and a width of 8 mm. The choane 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 choana, 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. 
684. Seen from above. 685. Lower aspect. 686. Right side. 
