120 ANATOMY OF THE WOOD RAT 



pear to be slightly more massive, relatively speaking; but 

 this may be only an optical illusion due to their greater size. 

 In that subgenus the coronoid process, upon which are in- 

 serted the temporal, is definitely more robust, thus con- 

 forming to other indications of the greater strength of this 

 muscle in the animal in question. The angular process, 

 upon which is inserted chiefly the internal pterygoid muscle, 

 is shortest in Homodontomys, conforming to the narrower 

 pterygoid fossa of this subgenus. Both the dorsal and 

 ventral masseteric ridges are equally well developed in all 

 three animals, their definition increasing with age. The 

 same may be said respecting the definition of the insertion of 

 the masseter superficialis upon the ventral border of the 

 angular process. 



The symphysis menti is movable as usual in the present 

 genus, and the two halves of the mandible are connected 

 by tough, elastic tissue. The fibers of the transversus man- 

 dibularis help to control their action and to lend additional 

 strength to their connection. As the muscle is thus of im- 

 portance, its attachment is marked upon either side by a 

 slight fossa, perhaps most extensive in Teonoma and least so 

 in Homodontomys. 



To recapitulate, direct indications upon the bones of the 

 head of comparative differences in the muscular develop- 

 ment of the three animals is as follows: The temporal 

 muscles are most powerful in Teonoma and least so in 

 Homodontomys. The infraorbital head of the masseter is a 

 trifle the largest in Teonoma and smallest in Homodontomys, 

 but probably the difference is not sufficient to be of func- 

 tional importance. The ectal and ental divisions of the 

 masseter major are apparently greatest in Homodontomys, 

 and least in Teonoma. The internal pterygoid muscles are a 

 trifle smaller in Homodontomys than in the other two sub- 

 genera. One or more of the three muscles attached to the 

 mastoid process of the squamosal is strongest in Teonoma, 



