i8o 



THE SKULL. 



[CHAP. 



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 infra- 

 orbital foramen, and through which a portion of the masseter 

 muscle passes (see Fig. 59). 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 lacrymal and malar behind. Its inferior boundary 



.Va 



\OSFrAS 



Per 



A a nasal; P\Ix premaxilla ; 3Ix maxilla ; Ma, malar; 7V tympanic ; ExO ex- 

 occipital ; Per points to the large supratympanic or mastoid bulla. 



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 (Lagostomus), the true infraorbital foramen 

 is separated from the large antorbital vacuity by a thin as- 

 cending bony lamella. In Castor, Leptis, Bathyergus, Sa'urus, 

 Arctomys and some others, the infraorbital foramen is of the 

 usual size. 



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

 reticulated. 



