1895.] 



59 



[Slade. 



la many of the rodents there is present a more or less extensive dilatation 

 of the infraorbital foramen, through wliich passes, in addition to the 

 nerve, that portion of tlie masseter muscle which has its insertion upon 

 the maxilla. This extends around the back of the jugal process of the 

 maxilla in a pulley-like manner to an insertion just below the socket of 

 the mandibular pra^molar, and thus cooperates with the temporal in 

 moving the mandible in a vertical direction. This attachment of a 

 head of the masseter is peculiar to the order, and explains the use of 

 the vacuity in the maxilla which oftentimes is of vast relative proportions. 



All existing rodents fall into two groups, the Simplicidentata and the 

 Duplicidentata. The first embraces the Sciuromorpha, Hystricomorpha, 

 Myomorpha, and the second, the Lagomorpha. 



In the Sciuromorpha, the jugal forms the greater part of the arch, ex- 

 tending forward to the lachrymal and posteriorly to the glenoid cavity, of 

 which it forms the outer wall, and is not supported below by a con- 

 tinuation backwards of the process of the maxilla. In the more typical 

 forms there is no enlargement of the infraorbital opening, while the 

 postorbital processes of the frontals are characteristic of the family 

 Sciuridie. The external pterygoid plate is entirely wanting and there is 

 no fossa. 



The arch in the Myomorpha is for the most part slender, and the jugal, 

 which does not extend far forward, is supported by the continuation below 

 of the maxillary process. The zygomatic process of the squamosal is 

 short. No postorbital process of frontal exists. The infraorbital opening 

 varies. In the family Muridiie, especially in the typical forms, this opening 

 is perpendicular, wide above and narrow below, while the lower root of 

 the zygomatic process of the maxilla is flattened into a thin perpendicular 

 plate. Very much the same condition exists in the Myoxidfe, while in the 

 Dipodidse, the foramen is as large as the orbit, is rounded, and has a 



