PETERSON : DESCRIPTION OF NEW RODENTS 161 
extends well back. he acetabular border is much more prominent than in 
Cynomys or Aplodontia. The pubic border is also quite prominent. The acetabu- 
lum is deeper than in the recent genera referred to; the ischium and pubis are 
- stronger, and the thyroid foramen is smaller. The pubic symphysis is short, like 
that of Aplodontia, and a detached epiphysis was found in position in the type, 
similar to that of Castor canadensis. . The vertical depth from the tuberosity on the 
ischium to the pubic symphysis is greater than in Aplodontia and Cynomys. The 
latter genera have the superior (anterior) part of the ilium turned outward (espe- 
cially Cynomys) at a considerable angle, while S. fossor has not this feature. This is 
due to the greater anterior extension of the ilium, beyond the sacrum, and in order 
to give space for the posterior lumbar region in the recent genera; while in S. fossor 
the attachment for the sacrum extends more anteriorly, the last lumbar being oppo- 
site the supra-iliac border. 
The Femur.— Both femora are well preserved in the type. This bone is some- 
what more lightly constructed, and the neck below the head is longer than in S. 
penimsulatus. Otherwise the similarity is very apparent in the two species. In 8. 
fossor the head is well rounded. The greater and third trochanters are relatively 
lighter, the whole bone longer, and not so flat as in the beaver. From Professor 
Scott’s description (“The Mammalia of the Deep River Beds,” p. 77) it appears that 
S. montanus has the third trochanter ‘ placed more proximally than in S. peninsu- 
? 
latus.’ In S. fossor the third trochanter is placed immediately above the middle of 
the shaft, as in S. peninsulatus. In 8S. fossor the fossa between the condyles is nar- 
rower and deeper than in S. peninsulatus. The condyles have the same relative 
proportion, and the trochlea are somewhat deeper than in the John Day species. 
In Aplodontia, the third trochanter is little developed, and is placed high up on the 
shaft like that in Cynomys. In Castor, the third trochanter is placed nearly mid- 
way between the proximal and the distal ends of the femur. 
Patella. —The patella is present in the type, and its characters are nearly like 
those in Cynomys and Aplodontia, but different from those in Castor. In the latter 
genus, this bone is thick superiorly, and tapers to a trihedral point inferiorly, while 
in S. fossor it has a more ovate outline, and is as heavy below as above. The bone 
is larger than in Cynomys and Aplodontia. 
The Tibia. —Vhe tibia and fibula are well represented in the type, being present 
on both sides. The tibia is 4 mm. shorter than the femur. In the beaver, it is 
over 20 mm. longer than the femur. In Fiber zibethicus the tibia and fibula are also 
much longer than the femur. In the fossil, the relative length of the femur and the 
shin-bone is more nearly similar to that of Cynomys and Aplodontia. Proximally 
