530 FOSSIL TURTLES OF NORTH AMERICA. 
On the lower surface of the skull (fig. 688) are to be noted the broad triturating surfaces 
of the maxilla, each presenting about the same relative amount of surface as in P. ferox. The 
transition between the triturating surface and the cutting-border of the jaw is rather abrupt. 
The choanal fossa is likewise formed somewhat abruptly. In the midline in front of the 
choanz may be seen the suture where the inner borders of the maxilla meet below the vomer. 
A portion of the vomer is exposed in front of this suture. The anterior palatine foramen is 
small, only 7 mm. long, while in the specimen of P. ferox referred to above, it is 11 mm. long. 
Fics. 687-689.—A myda? tritor. Skull of type. X 3. 
687. View of upper surface. 688. View of lower surface. 689. View of right side. 
The choanz are relatively small, being constricted as in Amyda cartilaginea and A. sinensis, 
and the constriction has been produced in the same manner in the fossil, as in living species. 
The basioccipital bone is rather broad, occupying nearly one-third of the width of the 
posterior part of the roof of the mouth, the lateral portions being formed by the pterygoids. 
This region is slightly concave both transversely and longitudinally. The articular surface of 
the quadrate is 20 mm. wide, from side to side, being relatively greater than in the case of P. 
jerox. The squamosal processes, formed by the backward extension of the squamosal and the 
paroccipital bones, are large and scroll-like. 
