THE PELVIS OF BURROWING MAMMALS 207 
The parallelism exhibited by these forms in the attainment 
of a horizontal pelvis which is firmly fused with the vertebral 
column and in which the symphysis is greatly reduced or absent 
is correlated with the mechanical force exerted by the hind 
limbs in propelling the body during continuous burrowing. This 
position of the pelvis transmits the locomotive force exerted by 
the hind limbs along a straight line from the articulation of the 
limbs to the anterior part of the body. The action of the flexor 
muscles is much greater than that of the extensor muscles, and 
consequently their attachments, posterior to the articulation of 
the hind limbs, have been developed at the.expense of the 
anterior portion of the pelvis. When this condition is attained, 
it is believed that the symphysis is a hindrance to the passage 
of the fetus at the time of birth, for the pelvis has become a 
closed box. The bones are so firmly codéssified dorsally and so 
necessary for the attachment of the large flexor muscles of the 
hind limb that it follows that the symphysis is lost in orerd to 
provide the necessary room for the passage of the fetus. 
The retention of the symphysis in the pocket gophers and a 
few others is attributed to the fact that in these forms the pelvis 
is very broad, while in the moles, shrews, and the meadow mice 
the pelvis is so narrow that the elimination of the symphysis 
has been more imperative. 
The broad and narrow pelves are correlated with two methods 
of digging. The pocket gophers stand with the hind legs wide 
apart and throw the earth back between them, while the moles, 
shrews, and the others with the narrow pelves push the earth 
to the sides and crowd their way through the soil. In the 
former case the pelvis is constantly drawn out laterally by the 
action of the muscles, while in the latter case the pelvis is 
compressed by the action of the hind limbs. 
A parallelism is also found in the crossing of the rectus ab- 
dominis muscles in the forms in which the symphysis is reduced 
or absent. This is believed to function as a support for the 
viscera in the absence of the symphysis. Exception to this cross- 
ing of the rectus abdominis muscles have been found among both 
the rodents and the insectivors which have lost their symphysis, 
