90 University of Califorma Publications in Zoology  [Vou.17 
defined in the three subspecies of canadensis than in subawratus or 
fiber. The proximal surface of the deltoid ridge has a tendency to 
be flattened transversely in leucodonta and phaeus, but not in sub- 
auratus, fiber, or belugae. The proximal origin of the deltoid ridge 
is more distally located in the three forms of canadensis than in sub- 
auratus or fiber (pl. 9). There is a marked difference in the form 
of the internal condyle; in fiber and in the three forms of canadensis 
it is greater in its longitudinal measurement than in swbauratus, and 
in subauratus its transverse dimension is greater than in fiber or in 
the three subspecies of canadensis. 
The supinator ridge is heavier and more inclined to have an an- 
terolateral ridge in subauratus and fiber than in the three subspecies 
of canadensis (pl. 9). In subauratus and fiber the supinator ridge 
forms an even curve, while in the three subspecies of canadensis there 
is an easily discerned angle about half-way between the shaft and the 
external condyle. 
There is a small tuberosity on the lateral anterosuperior surface of 
the trochlea in subauratus which does not exist in any of the other 
four forms (pl. 9, figs. 38, 39, 40, 41). The capitulum has a tendency 
to be heavier in belugae and phaeus than in subauratus, fiber, or 
leucodonta. 
In subauratus and fiber the anterior distal articulating surfaces 
are wider, in comparison with the greatest width of the distal ex- 
tremity of the humerus, than is the case with the three subspecies of 
canadensis. This is shown in the following table: 
Width art. Width dist. Ratio, 
surfaces extremity per cent 
Subspecies of Castor canadensis (three 
SP@CIMeENS))| sess sescsseccsccewecee seen seeeeee es 18.9 mm. 32.3 mm. 58.5 
Castor fiber (one specimen) wns PAULS) 31.0 67.0 
Castor subauratus (five specimens) ... 19.2 30.9 62.1 
The distal wall of the small ellipsoidal fossa on the posterior 
surface of the internal condyle is heavier in fiber and in the three 
subspecies of canadensis than it is in subauratus. 
The supinator ridge extends laterally farther from the distal 
articulating surfaces in canadensis than in subauratus or fiber (pl. 9). 
Number of differences noted between Castor canadensis and Castor 
subauratus, sixteen. Fiber conforms with swbhauratus in nine and 
with canadensis nm seven. 
