84 University of California Publications in Zoology  [Vou. 17 
and postzygapophyses inclined from the horizontal more than are 
the articulating faces in the three subspecies of canadensis. 
In subauratus and fiber, on the thirteenth thoracic vertebra, the 
anapophyses are heavier than the same processes of the three sub- 
species of canadensis. In the last-named species and fiber, however, 
they are longer than in subauratus. 
The average lateral extension of the metapophyses and ana- 
pophyses, measured from the lateral tip of the left to the lateral tip 
of the right process, and the ratios of the two widths at the thirteenth 
thoracie vertebra, are as follows: 
Ratio of widths 
of metapophyses 
Metapophyses Anapophyses to anapophyses 
Castor fibers se 21.88 mm. 33.74 65% 
Castor SUbaUnatIS, pecceseee eee 25.4 29.1 89 
Three subspecies of Castor cana- 
densis; 0.46). eee 24.2 30.5 79 
C7) 
YY 
Fig. M.—Dorsal view of second lumbar vertebra of Castor fiber, no. 19229. 
Fig. N.—Dorsal view of second lumbar yertebra of Castor subawratus, no. 
16385. X 3%. 
Fig. O.—Dorsal view of second lumbar vertebra of Castor canadensis belugae, 
no. 4347. X %. 
It will be noted from this table that the ratio of the width of the 
metapophyses is nearer unity in swbawratus than m the three sub- 
species of canadensis or fiber. 
In the lumbar vertebrae the transverse processes of the three sub- 
species of canadensis have a greater spread transversely than have 
those of subauratus (text-figs. M, N, O, P, Q, R). The shape of these 
processes also differs, the transverse processes in subawratus and fiber 
being nearly uniform in width from their origin at the pedicles to 
their extreme lateral portion; those of belugae taper near their lateral 
extremity, and in leucodonta and phaeus there is a sharp point at the 
very end (text-figs. M, N, O). 
The mesial edges of the postzygapophyses of the lumbar vertebrae 
in subauratus form an angle with each other, while in the three sub- 
