2 MONOGRAPHS OF XORTEI AMERICAN BODENTIA. 



Tin 1 character of the anteorbital foramen is peculiar, and probably diag- 

 nostic of 1 lie 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 interior 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 Hie 

 frontal bone . . ." 



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: 



MURINiE. — 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. 



ARVICOLINyE. — Molars normally rootless (except in Evotomijs), 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 



