94 University of California Publications in Zoology [Vou. 17 
At the distal extremity of the posterior face there is a process 
which has a much sharper point in subauratus than in the three sub- 
species of canadensis or in fiber; but this process is smaller in sub- 
auratus and fiber. 
Number of differences noted between Castor subauratus and Cas- 
tor canadensis, ten. Castor fiber conforms with subauratus in seven 
and with canadensis in three. 
FiBuna 
The angle which the hamular process (see Morgan, 1868, p. 54, for 
name) makes with the shaft of the fibula approximates, in Castor sub- 
auratus and Castor fiber, ninety degrees, and is decidedly less than 
ninety degrees in the three subspecies of Castor canadensis (see pl. 8, 
figs. 25 to 29). 
On the proximal third of the fibula the following differences occur : 
in the three subspecies of canadensis and in fiber the lateral face has 
a decided tendency to be concave, while in subauwratus there is a tend- 
ency toward convexity. In the three forms of canadensis and in fiber 
there is a well-defined posterior face which joins the medial face in 
a sharp-edged medial crest, the medial face being slightly coneave. 
In subauratus the medial and posterior faces are poorly defined and 
the medial crest is but a faintly indicated line (pl. 8, figs. 25 to 29). 
Proximad, and directly anterior to the face for the articulation of 
the tibia, the fibula in swhawratus and fiber presents a small, flat face, 
while in the three subspecies of canadensis there is a sharp crest at 
this point. 
The distal epiphysis has proximally pointing projections which are 
claw-shaped in the three forms of canadensis but are less developed 
in subauratus or fiber. 
On the proximal posterolateral margin of the hamular process in 
canadensis is a crest which does not occur in subauratus or fiber. 
On the posterior face of the distal epiphysis is a suleus which is 
wider in the three forms of canadensis and in fiber than in subawratus. 
The medial face of the external malleolus tends to be convex in cana- 
densis, while in subauratus and fiber this articulating face presents 
the appearance of having been ground and is concave. 
Number of differences noted between Castor subauratus and Cas- 
tor canadensis, eight. Castor fiber conforms with subauratus in five 
and with canadensis in three. 
