THE PELVIS OF BURROWING MAMMALS 205 
The occurrence of both types of pelvis in the females of the 
same species cannot be explained unless it is the case of a 
structure in the course of reduction, the pocket gophers repre- 
senting a stage in which the first step is being taken, while the 
mole-rats (Bathyergidae) have not yet reached this stage, the 
moles and shrews having passed on to the completed stage as 
shown by the absence of the symphysis in both sexes. 
4. The function of the crossed rectus abdominis muscles 
The relations of the rectus abdominis muscles show a general 
conformity to the condition of the pelvis. Lecke (’92) believed 
that the crossed conditions of these muscles was for the purpose 
of compensating the loss of the symphysis, giving support to 
the viscera. As stated above, he found the beginning of the 
crossing of the muscles where there was the first tendency toward 
the loss of the symphysis in the hedgehogs (Erinaceidae) and 
found the crossing of the muscles most complicated in the 
moles (Talpidae) where ‘tthe symphysis is entirely absent. The 
South African golden mole (Chrysochloris), constituting an 
exception to this, but in this form the gracilis muscles meet in 
the midline and, together with the rectus muscles, form an 
efficient support to the viscera. 
The rectus abdominis muscles of the pocket gophers have 
been shown to vary in their attachments, but the crossing of the 
muscles has accompanied the loss of the symphysis. In the 
pocket gopher (Geomys bursarius), a strong aponeurosis is 
formed which affords an efficient support to the viscera. Even 
though there is a variation in the muscles throughout the family, 
the provisions for the support of the viscera, by the crossing of 
the muscles or some other device, is constant. The fact that 
the muscles are crossed in some of the mice (Muridae) in which 
the symphysis is present would substantiate Lecke’s findings, 
that the muscles have a tendency to cross over in forms in which 
the symphysis shows the beginnings of reduction. 
Here, then, is another parallelism between the Rodentia and 
the Insectivora which may be referred to the similar necessity 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 25, NO. 2 
