a 
a 
A Skeleton of Great Fossil Beaver, Castoroides Ohtoensis. 141 
ciently fractured to show the dimensions of the pulp cavities. The 
vomer is preserved and shows the well-defined groove of the upper 
margin for four inches or more. 

35 
Fic. 3.—One-third natural size. Posterior, upper and left view of upper jaw: ¢, 
erowns of incisors; g, posterior groove of left incisor; mmo, posterior left molar; m, 
malar process; 70, infra-orbital foramen; s, ante-orbital sinus; ss, straw through 
upper posterior nares; si, straw through lower (triangular) posterior nares. Imme- 
diately above and on either side of this triangular opening may be seen the anterior 
portions of the pterygoid fosse. 
The large ant-orbital sinuses of the maxillary are well shown, 
through the bases of which the grooves pass backward from the 
infra-orbital foramina. Special attention has been given by Wyman, 
Allen and others to what is probably the most marked character- 
istic of the genus, namely, the double posterior nares, which 
results from ‘‘ the extraordinary development and peculiar confor- 
mation of the pterygoid processes.”’ Said fossze would be two 
and a half inches deep in this specimen if they were complete. As 
it is, there remains on either side about an inch of groove. These 
two grooves indicate the lower portions of the fosse. They extend 
forward and curve upward, giving us the bottoms of the fossz, 
reaching anteriorly. We are here shown perfectly how the 
inner walls of the processes curve inward along the median 
line, till they coalesce at the surface of their adjacent con- 
vexities, and thus divide the posterior nasal opening into an 
upper and a lower. The lower opening is nearly an equilateral 
triangle, with the posterior margin of the palatine as a base. The 
upper opening, spoken of by Prof. Allen as ‘‘ pyriform’’ (and 
