568 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



plane lengthwise, lnit much arched in the opposite direction. To the outer 

 side of the glenoid fossae, the zygomata form a broad llal expansion or horizontal 



plate, which widely separates the ends of the malars from each oilier, 

 and largely increases the zygomatic width of the skull, so conspicuous in 

 this species. Nothing of the sort is seen in Arctomys, Sciurus, etc., where 

 the zygomatic process of tin' squamosal bends downward from the outer 

 holder of the glenoid; while, in Haplodon, it forms a broad horizontal shelf 

 for articulation to the malar (?). At any rate, the broad plate is there, but 

 how much of it is squamosal and how much is malar, the obliteration of the 

 suture prevents me, from determining. 



The occipital view of the skull is Hat, more or less perpendicular, though, 

 especially in younger sknlls, with decided forward-upward obliquity, two or 

 more times as wide as high, with the general contour of a low isosceles triangle. 

 The straight ness of the basal line is chiefly broken by the downward projec- 

 tion of well-developed paroccipital processes: the upper outline curves as 

 already described in speaking of the superior view of the skulls. The general 

 surface is approximately plane as well as perpendicular, though appearing 

 depressed at the sides, owing to the projecting of the flange-like occipital 

 ridge. A considerable portion of the back walls of the bullae auditorial appears 

 upon this plane of the skull, as irregularly quadrilateral plates bounded 

 exteriorly by the mastoids, interiorly by the paroccipitals, and themselves 

 forming a part of the superior border of the occipital plane on each side. The 

 foramen magnum appears mostly in the plane of the occiput, its lower margin 

 merely making a shallow emargination of the base of the skull (more con- 

 spicuous in youngish than in old skulls). The perpendicular portion of the 

 orifice is subcircular, but somewhat broader than high; its upper semicir- 

 cumference is thin-edged, the rest being occupied by the condyles, the 

 articular surfaces of which are remarkably narrow for their length, and 

 closely approximate to each other inferiorly. 



As well as can be judged without actual measurement, the capacity of 

 tin; cranium is decidedly smaller, in comparison with the rest of the skull, 

 than in Sciurus proper, or even such a form as Cynomys; nevertheless, there 

 does not appear to be much difference in (his respect between the cranium 

 of Hajilodon and that of Arctomys. Viewed from the inside, the walls of the 

 brain-cavity show a decided impression for the cerebellum, distinguished by 

 an arched ridge from the cerebral impressions, while the petrosals offer a 

 large subcircular prominence with two conspicuous perforations. 



