

568 



MONOOIiAPHS OF NORTH AMEBIOAN BODENTIA. 



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

 side of the glenoid fossae, 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 Haplodon, it forms a broad horizontal shelf 

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

 how much of it is squamosal and how niuch is malar, the obliterutiun 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 a|>proximately 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 auditorise appears 

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

 exteriorly by the mastoids, interiorly by the paroccipitals, and them^lves 

 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). Tiic perpendicular portion of the 

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

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

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

 closely approximate to each other inferiorly. 



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

 the cranium is decidedly smaller, in com|>arison with the rest of the skull, 

 than ii Sciurus 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 

 un arched ridge from the cerebral impressions, while the petrosals oiTer a 

 larg'j 8ul>circular prominence with two conspicuous perforations. 



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