56 University of California Publications in Zoology  |Vou. 17 
SacRAL VERTEBRAE 
The term sacral vertebrae, as used throughout this paper, includes 
those vertebrae between the lumbar and caudal vertebrae which are 
firmly fused together and one or more of which are joined to the ilia. 
It is in this sense that the term is used by Owen (1866, p. 366), 
Morgan (1868, p. 51), and Flower (1876, p. 62); and this is essen- 
tially the definition given by Reynolds (1897, p. 16), although on 
page 452 of the same work he says that beavers have but one sacral 
vertebra. 
There are five points where fusion between the sacral vertebrae 
may occur: the lateral extremities of the transverse processes; the 
prezygapophyses and postzygapophyses; the pedicles; the neural 
spines; and the centra (pl. 6). 
Phaeus, belugae, and one specimen of lewcodonta (no. 12107) have 
the first four vertebrae following the lumbars fused at all five points 
(pl. 6, figs. 14, 15). The second specimen of leucodonta (no. 12101) 
has the first three fused at all five points, but the fourth is fused 
to the third at the zygapophyses and the neural spines only (pl. 6, 
fig. 16). From this it will be seen that there is a decided tendency 
toward four sacral vertebrae in the three subspecies of canadensis. 
In subauratus, however, three is the usual number (pl. 6, figs. 9, 
10, 11, 12). In none of the specimens of the latter are the centra 
of more than three vertebrae fused together (no. 16385 has only 
two), although no. 12668 has a peculiar dorsal epiphysis that joins 
the neural spines of the third and fourth vertebrae. The only 
specimen of subauratus which does not show three and only three 
sacral vertebrae is no. 8987, which has the third and fourth vertebrae 
fused at the neural spines and the pedicles. 
Leucodonta, no. 12107, has well-developed rudimentary ribs, such 
as are to be seen in Hrethizon, joming the second sacral vertebra to 
the ilia. This condition exists, but to a lesser degree, in nos. 210, 
4347, 19229, and 12101, of phaeus, belugae, fiber and leucodonta, re- 
spectively. Subauratus does not show these rudimentary ribs in any 
of the specimens (pl. 6). From this it will be seen that, if Gegenbaur’s 
definition (Flower, 1876, p. 25) for the sacral vertebrae were used, 
the three subspecies of canadensis would have two sacral vertebrae 
and subawratus one. 
The neural spine of the three subspecies of canadensis on the first 
sacral vertebra is almost perpendicular, while in subawratus and fiber 
this process inclines slightly caudad. 
