1917] Holden: Osteological Relationships of Beavers 79 
shows a very marked tendency to be greater in swhauratus and fiber 
than in the three subspecies of canadensis (see text-figs. D, E, F) ; the 
average width for the last three forms is 21.8 mm., for swbauratus 
26.1 mm., and for fiber 25.7 mm. The neural canal in the axis is 
Fig. D.—Cephalic view of axis of Castor fiber, no. 19229. X 1. 
Fig. E.—Cephalie view of axis of Castor subauratus, no. 12654. X 1. 
Fig. F.—Cephalie view of axis of Castor canadensis leucodonta, no. 12101. 
x 1. 
larger in subauratus than in the three subspecies of canadensis, the 
canal in fiber being intermediate in size (see text-figs. D, E, F). 
The neural canal in the third cervical vertebra of subawratus is 
nearly semicircular in shape, while in fiber and the three subspecies 
Fig. G.—Cephalic view of third cervical vertebra of Castor fiber, no. 19229. 
x I 
Fig. H.—Cephalic view of third vertebra of Castor subauratus, no. 8987. X 1. 
Fig. I.—Cephalic view of third cervical vertebra of Castor canadensis leuco- 
donta, no. 12101. X 1. 
of canadensis the laminae and pedicles form a segment of a circle 
which is greater than a semicircle (text-figs. G, H, 1). The vertebr- 
arterial canal of swbawratus in the third cervical vertebra is almost 
circular in shape, while in the three subspecies of canadensis the canal 
