78 University of California Publications in Zoology [Vou. 17 
COMPARISONS 
CERVICAL VERTEBRAE 
In all the cervical vertebrae there is a tendency for the laminae 
and the pedicles to be heavier in Castor fiber and Castor subauratus 
than in Castor canadensis belugae, C. c. leucodonta, or C. c. phaeus. 
The neural canal appears to be more constant in shape throughout the 
series of subauratus and in fiber than in the three subspecies of cana- 
densis. There is a tendency for the articulating face of the post- 
Fig. A.—Dorsal view of atlas of Castor fiber, no. 19229. X 1. 
Fig. B.—Dorsal view of atlas of Castor subauratus, no. 12654, X 1. 
Fig. C.—Dorsal view of atlas of Castor canadensis phaeus, no. 210. X 1. 
zygapophysis of subauratus and fiber to be more convex and to face 
laterad to a greater degree than it does in the subspecies of canadensis. 
The average anteroposterior width of the most dorsal portion of 
the atlas is greater in subauratus and fiber, the average for the three 
subspecies of canadensis being 7.8 mm., for subauratus 9.8 mm., and 
for fiber 8.9 mm. (see text-figs. A, B, C). In the three forms of cana- 
densis the transverse processes of the atlas are thinner and constitute 
a greater proportion of the entire transverse width than the same 
processes of swbauratus. The odontoid process of the axis is most 
highly developed both in diameter and in length in phaeus; sub- 
auratus and fiber are next in size in this particular, and belugae and 
leucodonta are both smaller (see text-figs. D, E, F). The width of 
the anterior articulating face, at the base of the odontoid process, 
