THE PELVIS OF BURROWING MAMMALS _ 19% 
izontal, the sacrum, along with the caudal vertebrae, is arched 
‘dorsally so that the pelvis is far from being a closed box. The 
red-backed mouse (Hvotomys grapperi) and the meadow mouse 
(Microtus pennsylvanicus) have no’ symphysis and the ventral 
margins of the pubic bones are nearly parallel in the adult 
meadow mouse (figs. 8 and 9). The pelvis is very nearly hori- 
zontal, but the direction of the pubic bones is quite oblique 
ventrocaudad. In the young meadow mouse the pubic bones 
converge posteriorly and form the symphysis at their points of 
contact. 
The house mouse (subfamily Murinae) has a short symphysis 
formed by the point of contact of the two converging pubic 
bones as in the young meadow mouse. The pelvis is also less 
horizontal than it is in the meadow mouse (figs. 10 and 11). 
Throughout, the mouse family (Muridae), with the exception 
of some of the meadow mice, the estate of the pelvis is quite 
different from that of the pocket gophers (Geomydae). In the 
former, the pubic bones converge posteriorly, forming a sym- 
physis at their point of contact (fig. 10), while in the latter the 
pubic bones diverge posteriorly and the symphysis, when it is 
present, is formed by the union of bony processes from each 
side (fig. 4). 
The pelvis of the ‘Quette vole’ (Myospalax), the only member 
of the family Spalacidae which has been studied, was examined 
in the flesh. The symphysis is very short (one-twenty-fourth 
of the length of the pelvis) and appears to be formed by the 
fusion of two long bony processes, such as that prevailing 
among the pocket gophers. The ventral margins of the pubic 
bones also suggest the condition among the pocket gophers in 
that they are quite parallel. 
The squirrels (Sciuridae), in so far as they have been ex- 
amined, seem to be quite uniform in the possession of a well- 
developed symphysis and having the pelvis inclined to form a 
large angle with the sacrum. They have, therefore, been con- 
sidered to be a group in which the pelvis is quite generalized and 
not of special interest in the present consideration. 
