GILBERT: ON THE SKULL OF XEROBATES UNDATA COPE. 145 
Fig. 2.—3kull of Xerobates undata Cope, from below; natural size. 
The premaxillaries are sharp at their lower border, which 18 
gently concave in side view. Seen from below the lower plane is 
very uneven, with the basisphenoid only flattened. 
The premaxillaries are straight on the edge below and arched to 
a point above. They form almost the entire floor of the anterior 
nares. They are convex transversely in the middle, and doubly 
convex in front. They have no articulation with the palatine; in 
front below there is a deep, round fossa for the reception of the 
beak of the mandible. The posterior nares at the posterior 
margins of the bones are rounded. The premaxillaries articulate 
posteriorly with the stout descending process of the vomer. The 
maxillaries have two cutting edges, the inner one with its plane 
much above the planeof the outer. They send upa broad process, 
thinned and narrowed above to join the prefrontal. The outer 
cutting edges increase in thickness froma sharp, serrate one, to one 
four millimeters thick above. The pterygoids, palatines and 
vomers together form a deep ascending channel, broadest a little 
in front of the palatine foramina; the channel is divided by a low 
ridge in the middle, which in the anterior part of the vomer is thin 
and sharp and curves downward. The posterior process extends 
from the outer cutting edge, instead of from the second, as in 
many turtles. It is thin, acutely angled, and extends slightly out- 
ward and downward below the basal plane, while the outer surface 
slopes at an angle of forty-five degrees. The grove on the inner 
