PETERSON: DESCRIPTION OF NEW RODENTS 143 
The parietals extend well forward and overlap the posterior part of the frontals. 
Laterally, they unite with the squamosals by a suture, which runs antero-posteriorly 
in a parallel line with the long axis of the skull. Posteriorly the parietals are wing- 
shaped, on account of the large triangular interparietals. From the base of the 
sagittal crest each ‘parietal is gently convex laterally. At the squamosal suture 
there is a slight swelling, the temporal region of the squamosal being more 
abruptly convex towards the zygomatic process than is the case in Aplodontia. 
The brain case is rather flat, but rapidly expands back of the supraorbital con- 
striction. , 
The interparietal is large, and is like that of the recent beaver. In this region 
the skull bears a great resemblance to that of Castor, with a comparatively broader 
occiput, and straighter lambdoid crest. Posteriorly, the interparietals are broad, and 
they taper rapidly to an anterior point, thus occupying the V-shaped space in the 
postero-superior border of the parietals. 
The occipital surface is almost vertical, as in Cynomys, but is much broader 
than in this genus. The supraoccipital occupies the greater portion of this area, as 
the mastoid is but slightly inflated. The entire surface superior to the foramen 
magnum is a vertical plane, and terminates superiorly in the lambdoid crest. In- 
feriorly there is a shallow emargination, which forms the superior border of the 
foramen magnum. At the point of contact with the exoccipitals in the lateral 
margin of the foramen magnum the suture extends outward and upward, and con- 
tinues in an almost straight line to the junction of the mastoid and the posterior 
process of the squamosal. The sharp lambdoid crest takes its origin a little below 
this point, and continues in a gently curved line to the sagittal crest. 
The occipital condyles are of moderate size and are not greatly separated by the 
emargination on the posterior face of the basioccipital. The paraoccipital process 
is small and points directly downward, terminating in a less truncated end than in 
the beaver. It is more nearly like that of Aplodontia. ‘The occipital condyle is 
close to the otic bulla. The mastoid portion of the temporal bone is very little in- 
flated. ‘The mastoid process is broad laterally, and compressed antero-posteriorly, 
extending proportionally as low as that of Castor. It unites with the strong mas- 
toid process of the squamosal, and is also fused together with the large external 
‘auditory meatus. This opening is somewhat like that of the beaver, the latter hav- 
ing a greater constriction just back of the opening of the tube. The whole region 
back of the zygomatic arches is short in S. fossor, and has a general resemblance to 
this region in Aplodontia. The tympanic bulla is inflated, flask-like, with a con- 
stricted but rather large tube, which is directed outward and upward, not hori- 
