PETERSON : DESCRIPTION OF NEW RODENTS 153 
heavy as that in the last lumbar. The centra of the posterior sacrals rapidly 
decrease in size, corresponding to the moderately long tail. 
The ilium is supported by the strong pleurapophyses of the three anterior sacrals. 
In the type specimen, the neural spines are all separated in a way similar to those 
in Cynomys. In:the cotype, however, the first and second spines are codssified. In 
, Aplodontia, the whole series back of the first spine is coalesced into a thin plate of 
bone, directed antero-posteriorly as also seems to be the case in Castor. In the type 
of S. fossor, the neural spine of the third sacral vertebra was injured during the proc- 
ess of cleaning up the specimen ; but the remains indicate, that it was perhaps the 
heaviest spine in the sacrum. The spinous processes on the two last sacrals are 
short, stout, and terminate in an enlarged rounded knob. 
The sacrum as a whole presents the aspect of a narrow, long, and vertically deep 
column of bones. In Aplodontia, the sacrum is of approximately the same length, 
but has much less vertical depth, and decreases more rapidly toward the posterior 
end, than in S. fossor. The ‘Sewellel” has a remarkably short caudal region, 
while S. fossor has one about the length of that of Cynomys. 
The Caudals. — As has been stated above, the tail of the type specimen consists 
of eleven anterior caudals. There were seven in a continuous series and in connec- 
tion with the sacrum. he four remaining vertebre are of about the right size to 
‘continue the series from seven to twelve. Assuming that the caudal series in 8S. 
fossor is similar to that of Cynomys, there would then be seven or eight vertebree 
missing at the end of the tail. 
The caudal region of S. fossor is heavier than in Cynomys, but in form it rather 
resembles the latter genus than Castor or Aplodontia. In the type the centra are 
short, round, and heavy. There is no neural canal back of the fourth caudal. The 
antero-posterior diameter is nearly equal in the first and last caudal vertebree in the 
specimen under discussion. In the five anterior caudals there are distinct transverse 
processes. In Cynomys, these processes can be traced back to the ninth caudal, 
while in Castor they continue to the end of the tail. From the sixth caudal to the 
tenth, there are small protuberances on the inferior faces of the vertebrae, which may 
or may not have supported chevrons. In the Trans. Amer. Philos. Soc., vol. 17, 
1893, p. 77, Professor Scott states that the anterior portion of the caudal region was 
provided with chevrons in S. montanus. The caudal vertebree present in the type 
(No. 1217) indicate a heavy, moderately long, and round tail. 
The Ribs, — Steneofiber fossor probably had thirteen pairs of ribs. Cynomys has 
twelve, Aplodontia fourteen, and Castor canadensis fifteen. 
In the type specimen, there is a fairly complete set of ribs. The first is much 
