THE PELVIS OF BURROWING MAMMALS 197 
and only a few of the fibers of the ventral layers of the muscles 
crossing over to the opposite side. In other cases the crossed 
condition is completely developed and the muscles originate on 
the bones at the sides of the pubic opening. Even the various 
subspecies of Thomomys bottae which have been examined 
differ as to whether the muscles originate on the bones forming 
the symphysis—in the absence of the symphysis on the pubic 
ligament—or on the bones at the sides of the pubic opening 
(fig. 22). 
Throughout the rodent group the tendency of the muscles to 
become crossed seems to accompany the loss of the symphysis. 
In the ‘mole rats’ (Bathyergidae) the muscles are only slightly 
crossed in the few specimens which have been examined. Among 
the mice (Muridae) the muscles are crossed in all the specimens 
examined which have no symphysis (fig. 23). Even the mice 
with the short symphysis formed by the converging pubic bones 
possess crossed rectus abdominis muscles which originate on the 
posterior border of the ischium (fig. 24). 
2. The relations of the rectus abdominis muscles of Insectivora 
The insectivors present a series of muscle variations as com- 
plete as the symphyseal variations themselves. The crossing of 
the rectus abdominis muscles accompanies the loss of the sym- 
physis in all the cases which have been examined (figs. 25 and 26) 
except the South African golden mole (Chrysochloris), in which 
case there is a specialized condition of the gracilis muscles, 
whereby they meet on the midventral line and form a firm 
union with the rectus abdominis muscles. 
Lecke (92) has studied the modification of the rectus ab- 
dominis muscles in the Insectivora, both as to their phylogeny 
and their ontogeny, and has concluded that their crossing is to 
be correlated with the loss of the pubic symphysis. He states 
that the beginning of the crossed condition is found in the 
‘hedgehogs’ (Erinaceidae) and the Gymnura and that it is 
mostly highly developed in the moles (Talpidae) and the shrews 
(Soricidae). Thus, the burrowing insectivors agree with the 
