1917] Holden: Osteological Relationships of Beavers 83 
There is a tendency for swbauratus and fiber to have ten clearly 
defined pairs of transverse processes, and for the three subspecies of 
canadensis to have only nine. One specimen of the latter possesses 
only eight. 
The neural spine of the first thoracic vertebra is shorter and less 
inclined from the vertical in suwbauratus than in fiber or the three 
subspecies of canadensis (pl. 5, figs. 1, 2, 3, 4). On the first thoracie 
vertebra of fiber, subauratus, and phaeus there is a small notch on 
the posterior margin of the vertebral arch at the base of the neural 
spine, which does not appear in the other two forms. It is possible 
to distinguish subauratus from phaeus or fiber by this character, as 
the notch in phaeus and fiber allows the neural spines of the first two 
thoracic vertebrae to come very close together along the median plane, 
while in subauratus the neural spines are held apart. 
There is a tendeney for the neural spine of the fourth thoracic 
vertebra to be longer in subauratus and fiber than in the three sub- 
species of canadensis. 
In the three forms of canadensis the ear-shaped processes on the 
eighth thoracic vertebra extend laterally to a greater degree than the 
transverse processes, while in subawratus and fiber the opposite is 
true. In subauwratus, leucodonta, and belugae, on the seventh and 
eighth thoracie vertebra, the right postzygapophysis is shorter than 
the left process. This is not true in phaeus or fiber, in which forms 
they are of equal length. 
There is a tendency for the rudimentary metapophysis to occur 
on the tenth, eleventh, and even on the twelfth thoracie vertebra of 
subauratus, while in belugae, fiber, and leucodonta this process is 
found on the ninth and tenth thoracie vertebrae, or on the tenth 
only. This process articulates with the postzygapophysis of the pre- 
ceding vertebra. The exact location of this process is as follows: 
subauratus, tenth and eleventh, nos. 12654, 12688, and 16385; eleventh 
only, no. 8987; eleventh and twelfth, no. 8869; lewcodonta, ninth and 
tenth, no. 12101; belugae and fiber, tenth only, nos. 4347, and 19229. 
This process does not occur, so far as the material at hand shows, 
in phaeus. 
On the tenth thoracic vertebra of phaeus there is a tendency for 
the postzygapophysis to bifureate and to have a dorsal as well as a 
ventral articulation with the prezygapophysis of the eleventh thoracic 
vertebra. : 
There is a tendency for the last four thoracic vertebrae of swb- 
auratus and fiber to have the articulating faces of the prezygapophyses 
