X.] RODENTIA. 157 



Hystrix their development is enormous, but this is chiefly 

 owing to their breadth and backward extension over the 

 great nasal chambers and air sinuses. They are narrowest 

 in Bathyergus and its allies. 



The premaxillae are large, and lodge the great curved 

 incisor teeth, ^ and always send a narrow prolongation back- 

 wards by the side of the nasals to join the frontals. 



In the larger number of Rodents there is a great vacuity 

 in the anterior or maxillary root of the zygoma of varying 

 size and form, apparently an enormous dilatation of the 

 infraorbital foramen, and through which a portion of the 

 masseter muscle passes (see Fig. 55). It is sometimes as 

 large as the orbit itself, with which it communicates freely 

 posteriorly, underneath a vertical bar, formed by the 

 maxilla in front, and by the lachrymal and malar behind. 

 Its inferior boundary is a slender zygoma-like horizontal 

 bar, formed by the maxilla alone. In the Rats it is a 

 vertical fissure dilated superiorly. In the Viscacha (Lagos- 

 fomus), the true infraorbital foramen is separated from the 

 large antorbital vacuity by a thin ascending bony lamella. 

 In Castor, Lepus, Bathyergus, Sciurus, Arctojnys, and some 

 others, the infraorbital foramen is of the usual size. 



In the Hares, the facial surface of the maxilla is curiously 

 reticulated. 



The zygoma is present in all, of various degrees of thick- 

 ness, but always either straight or more or less curved 

 downwards, and usually not much arched outwards. Its 

 anterior and posterior roots are formed by the maxilla 

 and squamosal respectively, the malar intervening. In 

 many cases the last-named bone extends backwards, applied 



^ These teeth, though first developed in the .gum covering the pre- 

 maxilla, have their roots, when fully developed, in the maxilla. This 

 does not invalidate their determination as incisors. 



