THE PELVIS OF BURROWING MAMMALS 203 
What has been said of the rodents which have a tendency to 
a reduction of the symphysis may also apply to the insectivors, 
such as the hedgehogs (Erinaceidae), which have a similar 
pelvis. Here again there is a correlation between the habits 
and structure which is parallel to the condition among the 
rodents, but a more extended study is necessary before making 
final statements. 
3. The persistence of the symphysis in certain cases 
There are certain cases. which seem, upon superficial exami- ° 
nation, to present exceptions to the explanations offered in this 
paper. Among the squirrel family (Sciuridae), the so-called 
ground squirrels, woodchucks, and a few others are considered 
fossorial mammals, and in all of these the symphysis is well de- 
veloped. However, all of these animals burrow only to make 
a shelter for themselves. Burrowing is a secondary matter 
with them and but a very small portion of their time is devoted 
to it. On the other hand, they are adapted to rapid running, 
for sitting erect, and for other activities which call for a pelvis 
very different from that found in pocket gophers and moles 
which are so restricted to continual burrowing. 
The mole rats (Bathyergidae) have many characteristics of 
well-adapted burrowers. Thomas (’85) considers them to have 
been specialized for a purely subterranean life. They are said 
to burrow in the sand and throw up the loose earth in piles with- 
out exposing themselves. Two of these forms were examined 
in the flesh (Bathyergus and Heterocephalus glaber) and the 
symphysis was found to be present, but, as explained above, it 
is very short and appears like a rod reaching between the two 
sides of the pelvis, a condition very similar to that found in the 
pocket gophers. 
Since the habits and structure are so similar to those of the 
pocket gophers, the presence of the symphysis may be explained 
in ‘the same way as that which is sometimes present in the 
pocket gopher is explained. The pelvis is‘broad and has allowed 
