THE PELVIS OF BURROWING MAMMALS 199 
vertebral column, and the line of locomotive force exerted by: 
the hind limbs passes directly along the vertebral column. 
The pocket gophers, moles, and shrews burrow continually in 
search of their food, and in so doing depend upon their fore 
limbs for propulsion. A pocket gopher, for example, digs the 
dirt loose and uses the hind limbs as a brace while so doing. 
The pelvis, parallel to the vertebral column and firmly co- 
ossified with it, offers the greatest advantage in directing the 
locomotive force along a straight line to the anterior part of the 
body. 
The firm union of the pelvis and the vertebral column is well 
illustrated by the marsupial mole (fig. 16) in which the co- 
ossification is complete and the structure is widely different 
from ‘that of other marsupials. Among the moles, Scaptochirus 
and Mogura (fig. 14) are also good examples of a rigid pelvis 
and sacrum. In these forms the iliopectineal prominences of 
the two sides have united to form a secondary symphysis dorsal 
to the viscera and just anterior to the articulation of the hind 
limbs. . 
Having seen that the pelvis of the mammalian ancestors was 
perpendicular to the vertebral column and that it became more 
nearly horizontal when the terrestrial habit was attained and 
finally that, in the mammals which had the habit of continual 
burrowing, the pelvis became horizontal, it seems most logical 
to conclude that these modifications are to be correlated with | 
the increased locomotive force exerted along the line of the 
pelvis and the vertebral column. The attainment of the hori- 
zontal position by the pelvis must be further considered in 
connection with the loss of the symphysis pubis. 
2. The loss of the symphysis pubis 
As the pelvis attains the horizontal position, the sacrum and 
the caudal vertebrae become more completely codssified and the 
symphysis moves posteriorly, probably due to the enlarged 
flexor muscles of the hind limbs which attach to the posterior 
portion of the pelvis, until it is formed entirely by the ischial 
