THE PELVIS OF BURROWING MAMMALS 195 
shrews, and meadow mice), in which the symphysis is always 
absent and the pelvis is very narrow, and the broad type (pocket 
gophers and a few others), in which the symphysis may or may 
not be present. This classification becomes clearer when placed 
in synoptic form as follows: 
Typical burrowing forms ( Narrow type of pelvis 
1. Ventral margins of pubic 1. Symphysis always absent. 
bones horizontal and di- 2. Pelvis narrow. 
verging caudad. 3. Includes: moles, shrews, 
2. Symphysis absent or, if pres- and meadow mice. 
ent, formed of a trans- 
~ verse rod of bone. Broad type of pelvis 
3. Includes: moles, shrews, 1. Symphysis may or may 
pocket gophers and cer- not be present. 
Pelvis of bur-} tain meadow mice. 2. If present, in form of a 
rowing mam-| 4. Figures 4 to 9 and 12 to 15. transverse rod. 
mals | Semi-burrowing forms 3. Includes: pocket gophers 
1. Pubic bones inclined caudo- | and a few others. 
ventrad, converging pos- 
teriorly to form a sym- 
physis at points of con- 
tact. 
2. Includes: mice in general 
and certain Insectivors. 
3. Figures 10 and 11. 
THE STATUS OF THE RECTUS ABDOMINIS MUSCLES 
Having considered the variations of the symphysis pubis 
and having found that in many cases it is absent, a consideration 
of the muscles which are typically attached to the bones forming 
the symphysis, the M. rectus abdominis, is very instructive. 
Typically, the rectus abdominis nruscle is paired, the right and 
left muscles lying on either side of the midventral line, with the 
origin on the pubic bones at the symphysis and the insertion 
on the first two or three ribs near their costal cartilages. The 
rabbit (fig. 18) illustrates very well the usual condition of these 
muscles near their points of origin. 
When the pelvis is modified by the loss of the symphysis, it is 
obvious that these muscles must also become modified, for the 
bone to which they normally attach is absent. It has been 
