t 
92 University of California Publications in Zoology [ Vou. 17 
The under surface of the ulna from the point of the olecranon 
to a point beneath the greater sigmoid cavity has, in the three sub- 
species of canadensis, the appearance of having been roughly modeled 
with a smooth, flat tool. This gives several smooth, flat surfaces, with 
more or less well-defined edges between them. In subawratus and 
fiber this surface is rounded, having no flat surfaces or well-defined 
edges. 
Immediately below the interosseous crest is a fossa which ends 
rather abruptly proximad in the three forms of canadensis and in 
fiber, while in subauratus it comes gradually to the surface (pl. 10). 
The minimum diameter of the shaft, which occurs near the distal 
extremity, is less in subauratus than in canadensis or fiber: subauratus 
measures 4.60 mm.; canadensis 5.17; fiber 6.50. In the three sub- 
species of canadensis the greatest diameter of the distal epiphysis 
is less in comparison with the total length of the epiphysis than in 
subauratus or fiber: the ratio of the diameter to length in canadensis 
is 76.1 per cent; subauratus 84.8; fiber 89.3 (pl. 10). 
Number of differences noted between Castor canadensis and Castor 
subauratus, ten. Fiber conforms with subawratus in six, with cana- 
densis in two, and two of the differences are unassigned. 
Os INNOMINATUM 
There is a tendency for the iliae portion of the acetabulum to be 
deeper and longer anteriorly in Castor subawratus than in the three 
forms of canadensis. Those portions of the ischium and pubis which 
form the base of the acetabulum are thicker in subauratus and fiber 
than in the three forms of canadensis. On the posterior dorsal edge 
of the acetabulum the lunate surfaces project farther ventrad in 
subauratus than in the three subspecies of canadensis, or in fiber 
(pl. 11). 
Ventral to the acetabulum is a square angle in fiber, subauratus, 
and phaeus, which does not oeeur in leucodonta or belugae. 
Number of differences noted between Castor subauratus and Castor 
canadensis, four. Fiber conforms with canadensis in two, and with 
subauratus im one. 
FEMUR 
In Castor subauratus the depression in the head of the femur is 
deeper than in the three subspecies of Castor canadensis or in Castor 
fiber. The diameter of this depression in subauratus and fiber is 
