1896.] MYOLOGY CF RODENTS. 167 
f rises from the whole length of the sternum; y (the abdominal 
fibres) are feebly marked; 6 (the part corresponding to the pec- 
toralis minor) comes from the lower true rib-cartilages and runs 
forward and outward to the head of the humerus and to the cora- 
coid process. Heteromys only differs from this description in 
haying the abdominal fibres more closely united with the panni- 
eulus than in any otheranimal. In the Gerbille, aand / are fused, 
while y and 6 unite near their insertion, which is into the capsule of 
the shoulder and into the humerus just internal to the pectoral ridge. 
The Hamster differs from the last only in having the abdominal 
fibres better developed and running quite separately to the cora- 
coid process. The Vole closely resembles the Gerbille, but is 
remarkable for the great development of the fibres from the 
cartilages (6); these fibres unite with y, and are inserted into the 
coracoid. 
In the Rat and Mouse the arrangement is almost identical with 
that of the Gerbille. 
In Siphneus Milne-Edwards describes a large and distinct pec- 
toralis minor inserted into the coracoid process and coming from 
the second rib; this is interesting when one notices the similar 
arrangement in the Vole, an animal to which he regards Siphneus 
as being more closely related than to the Mole-Rats *. 
Rhizomys resembles Cricctomys except that d is larger, rises from 
the 2nd to the 7th costal cartilages, and is inserted into the clavicle 
as well as the head of the humerus. Georychus bas a and / fused ; 
y is inserted into the neck of the humerus ; 0 rises from the 2nd, 
3rd, and 4th costal cartilages and is inserted into the coracoid and 
shoulder-joint. 
In Bathyergus the pectoral mass is very large, and resembles the 
last in having aand f fused; but near the insertion the posterior 
fibres become tucked under the anterior and are inserted deep to 
them into the pectoral ridge. The abdominal fibres (y) rise from 
the end of the sternum to the middle of the linea alba; they are 
inserted together with a large part of the latissimus dorsi into the 
capsule of the shoulder. The pectoralis minor (@) rises from the 
posterior two-thirds of the sternum and the posterior true rib- 
cartilages, and is inserted into the outer half of the clavicle, having 
its anterior border in contact with the posterior border of the 
subclavius; between these two contiguous borders the external 
anterior thoracic nerve emerges to reach the superficial parts of 
the muscle. 
Sterno-scapularis—In the greater number of the Myomorpha 
this muscle is only represented, as in Man, by the subclavius, which 
rises from the cartilage of the first rib and is inserted into the 
outer half of the posterior border of the clavicle. Georychus and 
Bathyergus are the only exceptions which I have met with ; in these 
there is a large and distinct claviculo-scapularis, as in the Hystrico- 
morpha. In Siphneus, according to Milne-Edwards, the arrange- 
ment is the same. 
1 «Recherches des Mammiféres,’ tome i. p. 99. 
