196 ROYAL NORTON CHAPMAN 
found that the rectus abdominis muscles show modification to 
be directly correlated with the variations of the symphysis. 
In the absence of the symphysis, the usual condition of the 
rectus abdominis muscles is a crossed one; the rectus muscle of 
the right side originating on the left pubic bone and the left 
muscle originating on the right. The two muscles cross a short 
distance anterior to their origin and proceed to their insertion 
in the usual manner (fig. 22). 
1. The relations of the rectus abdominis muscles of Rodentia 
Among the pocket gophers (Geomyidae), the rectus abdominis 
muscles show variations to be correlated directly with those of 
the pelvis. In the common pocket gopher of the Middle West 
(Geomys bursarius), the muscles spread out into a tough 
aponeurosis Just anterior to the pelvis when the symphysis is 
absent (fig. 19), while, in case the symphysis is present, the 
muscle fibers may be traced to their origin on the bones forming 
the symphysis (fig. 20). 
In the one specimen of Geomys hispidus which has been ex- 
amined, the muscles attached to the bones forming the sym- 
physis and are, in the main, uncrossed, although some of the fibres 
do appear to cross over to the opposite side. 
In Thomomys bottae bottae the muscles are crossed and 
attached to the bones forming the symphysis when the symphysis 
is present (fig. 21) or to the pubic ligament when it is absent. 
Thomomys bottae anitae possesses the same relation of the 
muscles as the preceding subspecies. In Thomomys monticola 
mazama the muscles are crossed and attached to the pubic 
ligament. Thomomys bottae pascalis and Thomomys fulvus 
intermedius possess crossed muscles which are attached to the 
bones at the sides of the pelvis in the absence of the symphysis. 
The pocket gophers, therefore, possess rectus abdominis 
muscles which, like the pelvis, are varying in their relations. In 
some cases the muscles exhibit only the rudiments of a crossed 
condition, the majority of the fibers composing the right and 
left muscles originating on the right and left sides, respectively, 
