1896.] MYOLOGY OF KODESTS. 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 {RMzoinys. Geori/chus, Balhyergus). 



Maxseter. — The four parts of the muscle already described are 

 present in the Myomorpha, but the posterior superficial and 

 posterior deep parts are usually difficult to separate satisfactorily. 

 The auterior superficial portion is constant and rises in front of 

 the infraorbital foramen, from the side of the maxilla, by a narrow 

 tendon. It is very strongly marked in the Spalacidae, 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 Georyclms and Bathi/erf/tts. 

 The anterior deep part shows an intermediate arrangement between 

 the Sciuromorphine and Hystricomorphine type and throws a 



Pig. 2. 



Ant.deep M^S. 



Sc/uromoT/lA/] 



Post. sup. ^^^St 



•Ant. sue MAS. ■ 

 Sii])erflcinl dissecdon of Ilanistor's nmsseter. 



good deal of light on the morphology of this portion. In the 

 Hystricomorpha there is a large infraorbital foramen through 

 which the antei'ior 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 dovifn 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 

 import-ince 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. In the 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 

 Pnoc. ZooL. Soc— 1896^ No. XI. 11 



