144 MEMOIRS OF THE CARNEGIE MUSEUM 
zontally outward as in Aplodontia. The position of the tympanic bulla is a char- 
acter which very strongly recalls that of the recent beaver. 
In the type (No. 1217) the base of the skullis damaged, but the cotype (No. 1208) 
supplements this region admirably. The basioccipital is entirely unlike that of the 
beaver, and is more nearly like that seen in Aplodontia and Cynomys. Anteriorly 
the basioccipital extends to opposite the anterior border of the otic bulla, in the type. 
In this region the inferior face has a slight shallow groove with two faintly indicated 
lateral ridges. These ridges meet 4 mm. back of the suture, and form a convex 
elevation, which increases in width posteriorly. In the beaver nearly the entire 
length and width of the basioccipital is deeply excavated inferiorly, which is a 
constant character. The presphenoid of the fossil is apparently similar to that 
of Castor. 
The pterygoids are prominent and terminate in backward projecting alee, which 
touch the otic bulle. Superiorly, these hamular processes are emarginated, so 
as to form a large, oblong foramen, which leads into the large pterygoid fossa. 
The outer process bounding the pterygoid fossa extends back to the antero- 
external face of the otic bulla, and unites firmly with the floor of the brain case. 
The foramen ovale pierces this outer process of the pterygoid near the extreme in- 
ferior border. The alisphenoid cannot be outlined in either the type or cotype of 
this species. Inskull No. 1212, however, the suture can be partly traced. ‘The bone 
appears to be of large size, and similar to that of the beaver. ‘The posterior wing 
extends to the tympanic, and forms a suture with it. The squamosal suture is im- 
mediately below the lower border of the glenoid cavity, and extends anteriorly ina 
sinuous line. The extreme anterior point cannot be ascertained, but may reach to 
the posterior border of the orbit. There is an alisphenoid canal. 
The squamosal occupies relatively a greater area superiorly, than in the beaver. 
It has also a larger posterior portion, which joins the supraoccipital directly. In the 
beaver the parietal has a descending lobe postero-laterally, which unites with the 
mastoid portion of the squamosal. ‘The squamosal is thus separated from the supra- 
occipital by this descending lobe in Castor. Anteriorly the squamosal extends rela- 
tively farther forward in S. fossor than in the beaver. ‘The zygomatic process 
of the squamosal is similar in size, shape, and position to that of Castor fiber. Pos- 
terior to and continuous with the base of this process is a prominent and sharp 
border, like that in S. peninsulatus, which according to Cope, enlarges the postglenoid 
fossa more than in either C. fiber Linnaeus or S. Hsert v. Meyer (S. viciacensis Gervais). 
There is a subsquamosal foramen present in the type. The glenoid cavity is an 
antero-posteriorly elongated and laterally convex surface. The antero-posterior 
