§2 University of California Publications in Zoology | Vou.17 
Hue, 1907, pl. 17, ‘‘S S’, hauteur totale du corps de la vertebre”’ 
““corps’’ does not correspond to the 
BNA term corpus), we find that anteriorly in the series the greater 
measurement is found in swbauratus and fiber (pl. 5, figs. 1, 2, 3, 4); 
(it should be noted that Hue’s 
on the first thoracic vertebra: swhauratus 16.1 mm., fiber 15.2 mm., 
and canadensis 14.3 mm. On the fourteenth thoracie vertebra, which 
has a larger measurement in all three species, the opposite order holds: 
canadensis 21.0 mm., subawratus 20.7 mm., and fiber 20.5 mm. Corre- 
lated with this is the extreme width of the thoracic vertebrae; in the 
earlier members of the series the greatest average measurement is found 
in fiber, next in width is subauratus, and the narrowest is canadensis. 
Later in the series, however, fiber, canadensis, subauratus, is the order 
(pl. 5). 
There are certain structural features which are differently located 
or differ in extent in the three species. The neural spines of the first 
ten thoracic vertebrae in suwbauratus and_ fiber are styliform, while 
the remainder are flattened from the sides; in the three sub- 
species of canadensis only the first nine are styliform, the remainder 
being laterally compressed. The thoracic vertebrae posterior to the 
eleventh or twelfth, in swhawratus, have an articulating surface be- 
tween the anapophysis and the prezygapophysis; on the other hand, 
there are no articulations between these processes in canadensis or 
fiber. 
On the dorsal surface of the transverse process of the first thoracie¢ 
vertebra of all three species of beaver so far as has been observed, 
there is a small tuberculum at the place where the anapophysis joins 
the metapophysis. In belugae, on the second thoracic vertebra and 
on several posterior to it, this process is ear-shaped (text-fig. L) ; in 
leucodonta this ear-shape appears first on the third thoracic vertebra ; 
in phaeus on the fourth; in subawratus and fiber this ear-shape does 
not occur on any of the first six (text-figs. J, K), but is seen first on 
the seventh thoracic vertebra. It is noteworthy that these differences 
show merely a tendeney in the three subspecies of canadensis for this 
ear-shaped process to occur farther anteriorly in the series of thoracic 
vertebrae. More material would be necessary to prove the exact loca- 
tion of the first appearance of this form of the joined processes in each 
subspecies of canadensis. 
In subauratus the neural spines of the thoracic vertebrae, with 
the exception of the first, are heavier than in the three subspecies of 
canadensis (text-figs. K, L). 
