504 MONOGRAPHS OP NORTH AMERICAN RODENTIA. 



gination anterior to the middle of the tooth, which is so strongly pronounced 

 on the anterior one as to produce a decided lobation. For the rest, the under 

 teeth are smaller than the upper, and especially narrower ; though the lengths 

 of the two series ( excluding the small upper premolar) are the same. 



There is no involution of the enamel sheet, nor any other complication 

 of the molar crowns, the island of dentine being surrounded with a wall of 

 enamel with the contour just described. 



C. DESCRIPTION OF THE .SKELETON. 



Skull. — The skull of Haplodqn is remarkable for its flatness or great 

 depression, its average depth (jaw and molars excluded) or dimension per- 

 pendicular to the horizontal longitudinal plane being little over one-fourth of 

 its length ; for its great width, especially behind the interzygomatic dimension, 

 being between three-fourths and four-fifths of its length ; and for the prepon- 

 derance, as Baird has remarked, "of horizontal planes and straight lines per- 

 pendicular to the three coordinate planes. Thus the plane of the occiput is 

 vertical and perpendicular to that of the base of the skull, which itself, as 

 nearly as may be, is horizontal. The general outline of the top of the head 

 is parallel with that of the bottom ; the lower edge of the occiput is horizontal, 

 and perpendicular to the horizontal [longitudinal] axis of the skull. The 

 lower edge of the zygoma is nearly rectilinear and parallel with the plane of 

 the palate. In tile lower jaw, a vertical plane would be tangent to the con- 

 dyles and the nearly horizontal posterior edg. A of the inferior ramus. The 

 planes of the broad coronoid processes are vertical, though inclined to each 

 other." Great as is the zygomatic width relatively to the length of the skull, 

 such width but little surpasses that of the occiput, owing to the great lateral 

 production of the mastoid and auditory bullae, the latter in some cases 

 extending beyond the mastoids. The skull is further remarkable among Sci- 

 uromorplta forms for the total lack* of postorbital processes, the interorbilal 

 constriction being much narrower than the rostrum, as in Arvicola, Geomys, 

 and many other Myomorpha. Aside from the absence of these processes, so 

 strongly characteristic of the Sciuromorph skull, the skull of Haplodon bears 

 a decided general resemblance to that of some Sciurines, as Arctomys, for 

 example; a resemblance so close, that much the same descriptive terras of 

 contour, &c, would imply to both, and the comparison might be pushed to 



* Shared, however, by Castor ; in .tiiumulurus, these processes are said to be "obsolete". 



