568 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
plane lengthwise, but much arched in the opposite direction. To the outer 
side of the glenoid fossze, the zygomata form a broad flat expansion or horizontal 
plate, which widely separates the ends of the malars from each other, 
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 the squamosal bends downward from the outer 
border of the glenoid; while, in Hapdodon, 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 flat, more or less perpendicular, though, 
especially in younger skulls, with decided forward-upward obliquity, two or 
more times as wide as high, with the general contour of a low isosceles triangle. 
The straightness 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 auditoriz 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 highs 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 
vlosely approximate to each other inferiorly. 
As well as can be judged without actual measurement, the capacity of 
the cranium is decidedly smaller, in comparison with the rest of the skull, 
than in Sciwrus proper, or even such a form as Cynomys; nevertheless, there 
does not appear to be much difference in this respect between the cranium 
of Haplodon 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. 
