198 ROYAL NORTON CHAPMAN 
other burrowing mammals in the possession of a pelvis which 
lacks a symphysis and in the peculiar eae condition of the 
rectus abdominis muscles. 
THE RELATION OF THE PELVIC ESTATES AND THE HABITS 
1. The attainment of a horizontal pelvis 
If the evidence offered by the probable ancestors of mammals 
is accepted, it must be considered that the pelvis was originally 
- confined to the ventral side of the body and served to articulate 
the hind limbs which pushed the body along over a parallel 
surface. Upon the attainment of a strictly terrestrial habit, 
the body was elevated above the substratum and the limbs 
necessarily functioned for support as well as for propulsion. 
This necessitated a firm union between the limbs and the frame 
work of the body. This was accomplished by the union of the 
pelvis and the vertebral column. 
Huxley (’75) has followed the series of changes from a condition 
in which the pelvis was nearly perpendicular to the vertebral 
column (fig. 1) to a condition in which it is nearly parallel (fig. 3). 
In the first instance the locomotive force exerted by the limbs 
must pass along a line extending dorsally through the pelvis (the 
iliac axis, Ja) and then anteriorly, almost at a right angle, along the 
vertebral column (the sacral axis, Sa). In the case of the mammal 
with the nearly horizontal pelvis, the force exerted passes along 
a nearly straight line, for the pelvis and the vertebral column 
are in nearly the same plane (the iliac and sacral axis nearly 
coincide). The mechanical advantage of such a structure is at 
once evident in the more direct application of the locomotive 
force to the framework of the body. 
The series of pelves of burrowing mammals presented in this 
paper agree in exhibiting the pelvis in every case more nearly 
parallel to the vertebral column than in any case described by 
Huxley (75). <A further modification is seen in the codossifi- 
cation of the sacral and caudal vertebrae and, in many cases, 
the codssification of these posteriorly with the ischium (fig. 6). 
This provides a still more firm union between the pelvis and the 
