200 ROYAL NORTON CHAPMAN 
bones. In the ordinary terrestrial habitat where the animals 
are free-ranging, an optimum condition is evidently reached, in 
which the force exerted by the hind limbs in propelling the body 
will be directed along the spinal column through the ilium which 
is placed at an efficient angle. But there are certain limitations 
in the attainment of a horizontal position of the pelvis. The 
symphysis may be brought so near the vertebrae that there will 
not be sufficient room to allow the passage of the fetus at the 
time of birth. When the caudal and sacral vertebrae are not 
coossified, the caudal vertebrae may be elevated in providing 
room, but in the event of their codssification the bones dorsal 
to the pelvic opening are rigid and the pubis, obviously, must 
provide the extra space needed for the passage of the fetus. The 
horizontal pelvis and the absence of the symphysis are therefore 
conditions which are interlinked, the latter dependent upon the 
former. 
There are two types of pelves among the burrowing mammals 
with regard to width, as has already been stated, and, in the 
broad pelvis of the pocket gopher, the symphysis sometimes 
persists, while in the narrow pelvis of the moles and shrews it 
is always absent. These two types of pelvis are to be correlated 
with two methods of burrowing. 
The pocket gophers have the habit of standing with the final 
feet wide apart while digging, so that the dirt may be thrown 
out between them. The moles, on the other hand, press the dirt 
to the sides and push it upward with the back—literally crawling 
between the earth particles. 
Seton (’09), in describing the habits of the western pocket 
gopher, wrote: 
‘‘TIts method of burrowing, as observed in a captive specimen, 
is to loosen the earth with the powerful front claws, as it stands 
with the hind feet advanced and wide spread, then throw it 
backward between the hind legs, to be further passed on by the 
hind feet; and when a sufficent pile is ready, the gopher turns 
round and pushes with its broad head and powerful front feet, 
forcing the pile ahead of it to the first gallery, up that and out, 
usually without exposing itself.” 
