1896.] MYOLOGY OF RODENTS. 161 
‘Thierreich,’ that the size of the temporal varies inversely with 
that of the eyes, for in them the eye is rudimentary while the 
muscle is very large (Rhizomys, Georychus, Bathyergus). 
Masseter—The four parts of the muscle already described are 
present in the Myomorpha, but the posterior superficial and 
posteriur deep parts are usually difficult to separate satisfactorily. 
The anterior superficial portion is constant and rises in front of 
the infracrbital foramen, from the side of the maxilla, by a narrow 
tendon. It is very strongly marked in the Spalacide, in which 
the lower jaw has a broad pulley-like groove under which the 
muscle passes to be inserted into the inner surface of the bone; 
this arrangement is very well seen in Georychus and Bathyergus. 
The anterior deep part shows an intermediate arrangement between 
the Sciuromorphine and Hystricomorphine type and throws a 
Fig. 2. 
.ANT.DEEP MAS. 
= Seruromorphine, 
YH), 
y KU Mo 
Post. sup, MAS... Z 
= Rew, ~ -- 
‘ANT.SUP.MAS, - 
/ 
{ 5G 
Zu 
Superficial dissection of Hamster’s masseter. 
good deal of light on the morphology of this portion. In the 
Hystricomorpha there is a large infraorbital foramen through 
which the anterior deep part of the masseter passes to be inserted 
by a narrow flat tendon into the mandible opposite, or just in 
front of, the anterior cheek-teeth. In the Sciuromorpha the part 
which I have hitherto described as “anterior deep ” rises from a 
vertical groove in front of the zygoma, and passes down to the 
same insertion without traversing any bony canal, the infraorbital 
foramen being only large enough to allow the passage of the 
nerve. As the insertion of a muscle is justly regarded as of more 
importance than the origin in determining homologies, I have 
hitherto looked upon these two portions as homologous, but the 
arrangement in many of the Myomorpha has made me change this 
opinion. Inthe Myomorpha the infraorbital foramen is inter- 
mediate between the arrangement found in the other two sub- 
orders; it is usually present as a vertical slit, through which 
passes a small bundle of muscular fibres corresponding to the 
anterior deep part of the Hystricomorphine masseter; but in 
addition to this there is another slip, corresponding to the Sciuro- 
morphine anterior deep part, which rises in front of the zygoma 
Proc. Zoot. Soc.—1896, No. XI. 11 
