570 



MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



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ill tire formation of the zygoma, vvhicli often affords, especially by its anterior 

 connections with maxillary or lachrymal, or both, valuable indications of 

 affinity among Rodents, remains unknown. I can discern no trace of the 

 sutures. I conjecture, however, from the line of fracture of an accrdentully 

 broken specimen, that the malar is anteriorly wedged ))etween two (anterior 

 and posterior) zygomatic processes of the maxillary, and extends nearly or 

 quite up to the lachrymal, — a supposition strengthened by the observed state 

 of the parts in Cynomys and Arctomys, tor example. Similarly, I am of 

 opinion that posteriorly the end of the malar will be found to represent the 

 prominent angle or corner of the zygomatic arch, being splinted underneath 

 a horizontal forward spur of the squamosal. The malar helps to define the 

 brim of the bony orbit by a small projection from its upper edge near the 

 fore end ; this is better marked in younger skulls, being obsolete or nearly so 

 in very old ones. With only aged skulls before him, Baird recognized a 

 lachrymal in a tubercle at the fore upper corner of the orbit ; in a younger 

 specimen, I sec nearly the whole contour of a large lachrymal, lying mostly 

 within the orbit, as a thin scale, thickening only at the ed^je to form the 

 nodule just mentioned ; it has a conspicuous canal. The suture of the 

 palatal plates of the maxillnries and palatines will probably be found opposite 

 the interspace between the last and penultimate molars. Anteriorly, the 

 maxillo-premaxiliary and fronto-maxillary sutures are preserved in one speci- 

 men ; the former encircling the rostrum below and laterally, then trending 

 obliquely backward as it mounts toward the forehead ; the latter being a short 

 backward-outward suture opposite the site of the lachrymal. Owing to the 

 narrowness of the nasals posteriorly, the premaxillaries gain the top of the 

 skull in a considerable area ; their suture with the frontal is a straight trans- 

 verse line continuous with the similar naso-frontal suture. Thus the frontal 

 bone ends squarely in a straight line across the forehead, with prominent lateral 

 angles for!i.<id by the fronto-maxillary sutures. The nasals, like the pre- 

 nuixillarios, arc perfectly distinct all around ; their shape has been already 

 noted. For the rest, the auditory bullae are the oiily other bones whose 

 whole periphery can be observed in the specimens before me; their remarkable 

 tubular prolongation from a globular base has been sufficiently described 

 above. 



Vertebral co/«/«».~Formula :— C. 7; D. 13; L. 6 ; 8.6; Cd. 11 = 42 

 vertei)r8e. In life, in a usual position, the backbone from atlas to end of 



