2 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
The character of the anteorbital foramen is peculiar, and probably diag- 
nostic of the group. “This,” as Baird has said, ‘‘consists of a narrow vertical 
fissure anterior to the corner of the frontal bone, widening above, and 
bounded externally by the zygomatic branch of the upper maxillary, which, 
instead of standing out more or less horizontally, is bent up, so that its ante- 
rior edge, at least, is almost in a vertical plane, and parallel with its fellow on 
the opposite side. In fact, the zygomatic process is divided into three parts: 
one articulating behind with the malar bone ; another completing the enclosure 
of the foramen just described; and a third articulating with the anteorbital 
process of the frontal. The inferior narrow part of the anteorbital foramen 
serves for the passage of the infra-orbital nerve; the wider upper portion, the 
outlet of which is directed upward, accommodates a portion of the masseter 
muscle. The suture of the malar bone, with the zygomatic process of the 
upper maxillary, is distinctly visible; the bone itself does not extend to the 
frontal bone a 
The two subfamilies with which we have to do on the present occasion 
are well contrasted, at least as far as North American genera are concerned, 
and may readily be distinguished by the following among other characters 
which might be enumerated : 
MURIN A8.—Molars rooted, tubercular, with crenate periphery.  Incis- 
ors compressed, narrower than deep. Root of under incisor causing a protu- 
berance on outer side of the mandible, at or near notch between condylar and 
coronoid processes. Descending process of the mandible a broad flattened 
plate, wholly below the plane of the molars. Anterior root of the zygoma 
deeply nicked at the anteorbital foramen. Zygoma (usually) dipping down to 
the level of the palate. ~ Palate nearly plane. Nasals projecting anteriorly. 
ARVICOLIN Ai. —Molars normally rootless (except in Evotomys), pris- 
matic, with flat crown and serrate periphery. Incisors often broader than 
deep. Root of under incisor causing a protuberance, if any, on the inner side 
of the mandible, at or near notch between condylar and descending process. 
Descending process of the mandible hamular; the apex of the hook attaining 
the level of the molars. Anterior root of zygoma not obviously nicked. 
Zygoma not dipping down to the level of the palate. Palate highly arched. 
Nasals not produced beyond premaxillaries. 
To the foregoing brief diagnostic characters, many points touching the 
general distinctions in outward appearance, habits, &c., of the groups might 
