1917] Holden: Osteological Relationships of Beavers 91 
Rapius 
The plane of the proximal articulating surface of the radius makes 
an angle of about ninety degrees with the shaft of the radius in the 
three subspecies of Castor canadensis, and in Castor fiber, while in 
Castor subauratus the superior margin of the proximal epiphysis is 
tipped distally, so that the plane of this articulating surface makes a 
more acute angle with the axis of the radius (pl. 10). The greatest 
diameter of the neck of the radius is less in the three subspecies of 
canadensis than in subauratus or fiber: canadensis averages 6.10 mm. ; 
subauratus 7.94; fiber 7.65. On account of this greater constriction 
at the neck of the radius, the head rises more abruptly from the shaft 
in the three subspecies of canadensis. The most lateral portion of the 
head bears a small tubercle in the three forms of canadensis which, in 
subauratus and fiber, is not so well developed (pl. 10). Proximad 
and mediad to the bicipital tuberosity is a well-marked fossa which is 
developed to a greater degree in subauratus than in fiber or in the 
three subspecies of canadensis. In the three forms of canadensis the 
sigmoid cavity is deeper than it is in subauwratus or fiber. 
Number of differences noted between Castor canadensis and Castor 
subauratus, six. Castor fiber conforms with subauratus in four of these 
and with canadensis in two. 
ULNA 
On the external face of the olecranon, near the posterior edge, 
there is a well-defined crest in the three subspecies of Castor cana- 
densis. In Castor subauratus and Castor fiber this is not developed 
to such a degree, but is seen as a line rather than as a crest (pl. 10). 
The anterior edge of the olecranon is thinner in the three sub- 
species of canadensis than in subawratus or fiber. This is in part 
due to the increased depth of the fossa located on the internal surface 
of the ulna below and proximad to the greater sigmoid notch. This 
fossa, in the three forms of canadensis, lies close to the superior edge 
of the olecranon, while in subauratus it lies farther posteriorly from 
this edge. The mesial edge of the coronoid process is thicker in sub- 
auratus and fiber than in the three forms of canadensis; in the latter 
this edge is sharp. The lateral edge of the lesser sigmoid cavity is 
also thicker in the case of subauratus and fiber than in the three sub- 
species of canadensis. Laterad to the lesser sigmoid cavity is a tuber- 
culum which is less developed in subauratus and fiber than in the 
forms of canadensis. 
