PETERSON ;: DESCRIPTION OF NEW RODENTS 145 
diameter is relatively greater than that of the beaver. The skull is broadest across 
the zygomatic processes of the squamosuals. 
The jugal is the most prominent bone in the zygomatic arch. ‘The anterior 
process does not reach the lachrymal, and is proportionally shorter than in the 
beavers. ‘There is a strong postorbital angle, similar to that in Castor fiber, S. vicia- 
censis (see Cope, Tertiary Vertebrata, p. 841), and S. peninsulatus. The anterior part 
of the jugal, across the postorbital process to the inferior margin of the arch, is the 
widest surface of the bone. From this point backward, the jugal decreases rapidly, 
and terminates in a somewhat enlarged end, which forms the exterior border of the 
glenoid cavity. Asa whole the zygomatic arch is proportionally wider and stronger 
than that of Castor, but otherwise is very similar. 
The palatine plate of the maxillary extends posteriorly to the line between mt 
and m*, and anteriorly to the posterior margin of the incisive foramina. In the 
beaver, the palatine plate of the maxilla is shorter posteriorly. In the type the 
posterior narial opening is well back of m#, and the front part of the palatine plate 
is much more concave than in Castor. Anterior to the alveolar border is a promi- 
nent ridge, extending from p* ina curved line on the side of the muzzle, terminating 
abruptly below and in front of the infraorbital foramen. In the beaver, this ridge 
is not so prominently extended to p*; it is heaviest at the infraorbital foramen, and 
continues obliquely upward, across the maxillo-premaxillary suture nearly to the 
top of the skull, and unites with the malo-maxillaryv ridge forming a deep trough- 
like fissure for the masseter muscle. In S. fossor the malo-maxillary ridge extends 
forward only a short distance and does not join the upward extended ridge men- 
tioned in the skull of Castor. In the type (No. 1217), the infraorbital foramen is 
small, which is characteristic of the genus. The zygomatic process of the maxillary 
is very strong, vertically broad, and ends abruptly at the inferior Jjugo-maxillary 
suture. 
The lachrymal cannot be outlined in the type or cotype. In skull, No. 1207, 
however, this bone forms an extremely small part of the facial region. The bone 
is very small, perhaps even comparatively smaller than in the beaver, and, as 
in that genus, is located immediately internal to the opening of the lachrymal 
foramen. 
The premaxillary is rather heavy to support the very large and powerful in- 
cisors. Below, it is laterally convex, with a median ridge extending from the in- 
cisive alveolar border to the anterior edge of the incisive foramen. ‘The latter is 
relatively broader than in Castor. The maxillo-premaxillary suture in S. fossor, 
may be traced from the posterior border of the incisive foramen in a slightly curved 
