80 University of California Publications in Zoology  [Vou.17 
tends to be elliptical, with the major axis extending transversely 
(text-figs. G, H, 1). There appears to be a tendency in belugae and 
phaeus for the vertebrarterial canals to persist throughout the entire 
seven cervical vertebrae; in lewcodonta they end with the sixth; in 
subauratus there are two foramina, as a rule, in the sixth cervical 
vertebra, while the seventh bears one in the right transverse process 
only; and in fiber, in the one specimen observed, there was one fora- 
men in the left process of the seventh thoracic vertebra. From this 
it will be seen that lewcodonta alone accords with the specimens of 
canadensis described by Morgan (1868, p. 51) in having foramina 
throughout six of the series only. 
Number of differences noted between Castor subauratus and 
Castor canadensis, thirteen. Fiber conforms with subawratus in nine 
of these, and with canadensis in one; three remaining unassigned. 
THoracic AND LuMBAR VERTEBRAE 
Castor fiber and the three subspecies of Castor canadensis follow 
the usual rule among rodents and the recorded descriptions of beavers 
in having nineteen trunk vertebrae (Owen, 1866, pp. 364, 365; Flower, 
1876, p. 50; Morgan, 1868, p. 51; Reynolds, 1897, p. 19). With the 
exception of one specimen, lewcodonta, no. 12101, they consist of four- 
teen thoracic and five lumbar vertebrae (Morgan, 1868, p. 19); the 
individual designated, however, has thirteen thoracic and six lumbar 
vertebrae, which, according to Owen (1866, p. 365), is generally the 
case among rodents, and holds with but few exceptions. It is to be 
noted, however, that a specimen of lewcodonta (no. 12107) has four- 
teen thoracic and five lumbar vertebrae, so the possession of thirteen 
thoracic vertebrae cannot be used as a character of lewcodonta. 
In the series of swbauratus, without an exception, the number of 
trunk vertebrae is twenty. In the nine skeletons of this species at 
hand, there are four with fifteen thoracie and five lumbar vertebrae ; 
four with fourteen and six; and one (no. 16382) with fourteen well- 
developed ribs articulating on the fourteen thoracic vertebrae, and a 
rudimentary rib on the right side of the first lumbar vertebra. It 
may be worthy of note, also, that of the five males three have fifteen 
thoracic vertebrae, the exceptions being nos. 16383 and 16385; and 
among the four females three have fourteen thoracic vertebrae, one 
of these having the variation noted above (no. 16382), and no. 8988 
with fifteen thoracie vertebrae. This may show that there is a tend- 
