112 MONOGRAPHS OF NOltTn AMERICAN RODENTIA. 



insr, however, those of Sigmodon, though falling short of those of Hesperomt/s 

 proper. Exceeding in size any other known North American species of Hes- 

 peromyS) and with the general appearance of Sigmodon or even Mus. 



Further details of this interesting section, t he most conspicuous among 

 North American Hesperomys, and almost worthy of generic rank, will be found 

 under the head of its typical and only known species, the common "Rice-field 

 Mouse" of the Southern States. While it does not require for its identifica- 

 tion any comparison with its allies, we may here note the position it holds 

 among them, after a description of the skull. 



Skull (Nos. mi, Utt, from South Carolina).— The size of the skull 

 alone distinguishes it from that of any other North American Hesperomys, 

 except, perhaps, H californicus, measuring nearly an inch and a quarter in 

 length by nearly two-thirds of an inch in zygomatic breadth. It has, how- 

 ever, other interesting peculiarities. Prominent among them is the sharp ridge 

 or bead into which the superior edge of the orbit is produced. However 

 sharp the border of the orbit may be in the smaller Hesperomys, it does not 

 form this bead, which seems to characterize chiefly larger forms, as we find it 

 in Mus decumanus, Sigmodon hispidus, &c. The anteorbital foramen tends 

 to assume a rather unusual shape for a Sigmodont, being subcircular above, 

 and running into a narrow slit below ; the feature, however, is not very strongly 

 marked, and may vary, moreover, with different individuals. We have seen 

 skulls of Mus decumanus in much the same condition. As in Mus, Sigmodon, 

 and Hesperomys proper, the incisive palatine foramina are long, reaching to or 

 even beyond a point opposite the anterior molars; the opposite is shown in 

 Ncotoma, where the foramina fall far short of the molar series. The great 

 backward production of the bony palate that Oryzomys shows does not occur 

 in apy other North American Hesperomys (where the posterior edge of the pal- 

 ate is about opposite the last molar), and perhaps represents one extreme in 

 this respect, of which Neotoma, where the palate is scooped out to opposite 

 the middle molar, may be the other extreme. The little deep pit, or fossa, on 

 either side of the palate opposite the last molar, is as in Sigmodon; but in the 

 latter these pits are separated by a median lengthwise process, which is not 

 the case in Oryzomys, where the hind border of the bony palate is almost 

 perfectly straight crosswise. This backward extension of the palate in Ory- 

 zomys is almost exactly the same as in Mus {decumanus); and, altogether, the 

 cranial characters, saving the Sigmodont dentition, are certainly the nearest to 

 those of typical Mus of any Hesperomys of North America, if indeed they 



