2 



MOXOGRAPIIS OF NOUTQ AMERICAN RODENTIA. 



The character of tlie antcorbital foramen is peculiar, and probal)!}- dia;,'- 

 nostic of the group. "This," as Baird has said, "consists of a narrow vortical 

 fissure anterior to the corner of the frontal bone, widening above, and 

 bounded externally by the zygomatic branch of the upper maxillary, wiiich, 

 instead of standing out more or less horizontally, is l)ent 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 <livided into three parts : 

 one articulating behind with the malar bone ; another completing the enclosure 

 of the foramen just described; and a third arlicuh'ting with the antcorbital 

 process of the frontal. Tiie inferior narrow pari of the antcorbital 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 massetcr 

 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 ..." 



The two subfixmilies with which we have to do on the present occasion 

 arc well contrastci'., 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, tul)ercular, with crenate periphery. Incis- 

 ors compressed, narrower than deep. Root of under incisor causing a protu- 

 berance on outer side of tiie niandible, 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 tiie zygoma 

 deeply nicked at the antcorbital foramen. Zygoma (usually) dipping down to 

 the level of the palate. Palate nearly plane. Nasals projecting anteriorly. 



ARVICOLIN^E. — Molars normally rootless (except in Evotomys), pris- 

 matic, with flat crown and serrate pcripliery. Incisors often broader than 

 deep. Root of under incisor causing a protuberance, if any, on the inner side 

 of tiie mandible, at or near notch between condylar and descending process. 

 Descending process of the ir.andil)le hamular; the apex of the hook attaining 

 tiie level of tiie molars. Anterior root of zygoma not obviously nicked. 

 Zygoma not dipping down to the level of the palate. Palate highly arched. 

 Nasii'.s not produced beyond premaxillaries. 



To the foregoing brief <iiagnostic characters, many points touching the 

 general distinctions in outward appearance, habits, &c., of the groups might 



