1917] Holden: Osteological Relationships of Beavers 87 
With but one exception (no. 12654), swbauratus and fiber have 
thickened lateral extremities on the transverse processes of the second 
and third sacral vertebrae, while the extremities of these processes in 
the three subspecies of canadensis come to a thin edge (pl. 6). 
Number of differences noted between Castor subauratus and Castor 
canadensis, four. Castor fiber conforms with canadensis in two, and 
with swbawratus in two. 
CaupaL VERTEBRAE 
It has been impossible to ascertain the number of vertebrae in the 
caudal region of several of the specimens, since some segments have 
been lost from the caudal extremity. The following specimens have 
their full complement of caudal vertebrae : 
Caudal Sacral 
No. vertebrae vertebrae 
Cas tonmfib eri ssstee st nsec 19229 25 4 
Gwen leucodonta: 2s..---: ease 12107 26 4 
C. subauratus (adult) ............ 12654 26 3 
C. subauratus (adult) -.... --- 16385 27 3 
C. subauratus (juvenile) 16384 28 3 
It will be seen that the number of caudal vertebrae is not constant 
in subauratus. The specimens described by Morgan (1868, p. 51), 
probably Castor canadensis canadensis, and Flower (1876, p. 67), 
probably the same species, had twenty-five caudal vertebrae. 
The lateral extension of the postzygapophyses of the first three 
caudal vertebrae is greater in subauratus than in the three subspecies 
of canadensis. There is also a tendency for the prezygapophyses in 
subauratus to have greater lateral extension than those in fiber or 
canadensis. 
Only the first four caudal vertebrae of swbauratus bear neural 
spines, while in phaeus and fiber the first seven, and in lewcodonta and 
belugae the first six have this process. 
The postzygapophyses of the first five or six of the caudal vertebrae 
project posterior of the centra in subauratus and fiber, but in the three 
subspecies of canadensis these processes project beyond the centra on 
only the first three. 
There is a tendeney for the prezygapophyses and the postzyga- 
pophyses of subauratus and fiber to be more nearly perpendicular to 
the dorsoventral axis, while the three subspecies of canadensis have 
these articulating faces set at an angle to this axis. 
