1894.] MYOLOGY OF RODENTS. 253 



an inferior from the zygomatic arch. I have verified this in 

 Arctomys, and find the description applies to all the Sciuromorpha. 

 In the Hystricomorpba it is difficult to satisfactorily separate the 

 upper and middle portions. 



In those animals, such as Dipus, Chinchilla, and Xerus, where 

 the posterior part of the skull is broad, the two temporals do not 

 meet in the mid line above. 



Masseter. — For the purposes of description it is most convenient 

 to divide this muscle into four parts — anterior and posterior supei"- 

 ficial, and anterior and posterior deep. These parts do not always 

 show a distinct line of demarcation. 



The anterior superficial part rises by tendon from the side of 

 the maxilla, and is inserted into the lower border and internal 

 surface of the mandible, extending up to the insertion of the 

 internal pterygoid. The posterior superficial rises from the whole 

 length of the zygomatic arch, and is inserted into the lower part 

 of the external surface as well as the lower border of the mandible. 

 The anterior deep part differs in the Hystricomorpha and the 

 Sciui-omorpha. In the former, among which the Dipodidse are 

 included, this portion rises from a large area on the side of the 

 maxilla, and then passes backwards and do^vnwards through the 

 enlarged infraorbital foramen to be inserted by a narrow flat 

 tendon into the alveolar margin of the mandible, external to the 

 molar teeth. In the Sciuromorpha this part of the muscle rises 

 from the top of a vertical groove in front of the anterior portion 

 of the zygomatic arch, the muscle runs down in the groove with- 

 out passing through the infraorbital foramen and is inserted as in 

 the Hystricomorpha. 



The posterior deep part rises from the lower border and some 

 of the internal surface of the zygoma, and is inserted into the 

 greater part of the external surface of the ramus of the mandible. 

 The arrangement used here is practically the same as that adopted 

 by Meckel, the only difference being that he describes the whole 

 superficial part under one name {jugo maxillien), although he states 

 that the anterior border has a very strong superficial tendon ; this 

 anterior tendinous portion I have found to be easily separable 

 from the rest in the Hystricomorpha, while in the Sciuromorpha 

 it is separated by a distinct interval. 



Buccinator.— This muscle is always well developed in Eodents, but 

 in most eases shoM's no special points of interest. It rises from the 

 maxilla and mandible opposite the molar and premolar teeth, and 

 running forwards blends with the orbicularis oris. 



In Spermnpliilus it is produced into the long cheek-pouch which, 

 when empty, hes folded back on the cheek, having a muscular 

 slip passing from the end of the pouch to the region of the 

 shoulder. 



Facial Muscles.— The orbicularis palpebrarum is not very strongly 

 developed ; from the anterior margin of it a muscle rises by a 

 narrow origin, but spreads out to be inserted into the upper lip 

 blending with the orbicularis oris, it probably represents the 



