1917] Holden: Osteological Relationships of Beavers $1 
eney toward sexual variation in subawratus in this regard. The proof 
of this tendeney would require for demonstration a large series of 
subauratus. 
Beside the differences recorded above, there are others which are 
seen when the thoracic and lumbar vertebrae are taken as separate 
eroups. There is a marked tendency for fiber and the three subspecies 
of canadensis to have larger centra in all three dimensions, in the 
thoracic vertebrae, than has swbawratus (text-figs. J, K, L, and pl. 5). 
This character is accompanied in the first thoracic vertebra by a 
broader neural canal in subauratus and fiber than in canadensis (pl. 
5, figs. 1, 2, 3, 4) ; although the height of the canal is also greatest in 
subauratus, 1 fiber it is similar to that in canadensis. In the verte- 
J K [E 
Fig. J.—Lateral view of fourth thoracic vertebra of Castor fiber, no. 19229. 
x1. 
Fig. K.—Lateral view of fourth thoracic vertebra of Castor subauratus, no. 
16385. X 1. 
Fig. L.—Lateral view of fourth thoracic vertebra of Castor canadensis belugae, 
no. 4347. X 1. 
brae which follow, however, there is a marked tendency for phaeus 
to have the largest neural canal (pl. 5, figs. 5, 6, 7, 8). In the eighth 
thoracie vertebra and the thoracic vertebrae which follow, phaeus 
possesses the largest neural canal, both in transverse and dorso-ventral 
measurements. 
The ratio of the width to the height of the eighth thoracic vertebra 
averages 83.3 per cent in subawratus, 84.5 per cent in fiber, and 69.7 
per cent in canadensis. On the thirteenth vertebra these ratios are: 
subauratus, 66.4 per cent; fiber, 100.9 per cent; and canadensis, 58.6 
per cent. 
If we compare the height of the thoracic vertebrae from the base 
of the neural spine to the most ventral portion of the centrum (ef. 
