530 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



keystone-shaped ;m<l straight, broad behind to articulate with both parietals, 

 narrowing anteriorly with nearly straight edge, and in front irregularly trans- 

 verse to accommodate its several rostral sutures. It sends a sharp spur on 

 either hand into a recess between the maxillary and intermaxillary, and each 

 intermaxillary causes a shorter, more obtuse re-entrance; the middle part 

 articulating with the nasals is transverse. The orbital portion of the frontal 

 is a simple plate suturing behind with the orbi to-sphenoid and squamosal, and 

 in front with maxillary and lachrymal. The latter is of considerable extent, 

 and much excavated ; its edge appears upon the surface of the skull, margin- 

 ing the back of the zygomatic process of the maxillary. 



The malar is very peculiar. In allied Rodents, this bone is a stout arch, 

 and very short, in consequence of the close approximation of the ends of the 

 zygomatic processes of both maxillary and squamosal. But here there is, to 

 all intents, no such squamosal process, and that of the maxillary ends abruptly ; 

 so the malar is a long bone, to complete the arch ; it is a straight rod, of 

 thread-like tenuity, with the fore end slightly elbowed and sharpened to lie 

 by oblique suture against the maxillary, on the inner side, and the hind end 

 slightly clubbed to suture by a roughened flat surface with the heel of the 

 squamosal ; and such are the relations of the parts that the malar seems to 

 run against the otic capsule. The depressed position of this bone, which lies 

 down on the level of the palate, has been already noticed. 



The maxillary bone, with a general shape and connections which scarcely 

 require notice, has two peculiarities; one of these is the singular position of 

 the " ante-orbital" foramen — here a large circular perforation at the anterior 

 border of the side of the bone, altogether remote from the orbit. It lies 

 above and even in advance of the incisive foramina. Its fellow is only separ- 

 ated by the width of the compressed muzzle; there is a thin partition, proba- 

 bly ethmoidal, between them. The other peculiarity of the maxillary is the 

 unusually extended and vaulted zygomatic plate, which thus roofs over a con- 

 siderable part of the orbit. This inflated lamina suddenly comes to a point 

 where the malar joins it. Its suture with the frontal, or the surface of the 

 skull, is a straight line. 



The incisive foramina, in Geomyidce, are wholly in the intermaxillaries ; 

 in Dipodomys they are formed by both bones, the maxillaries bounding about 

 a third of their periphery. The nasal spur of the intermaxillary extends upon 

 the forehead a little way beyond the ends of the nasals ; while a sharp pro- 



