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112 



MONOGUAPnS OP NOKTn AMERICAN RODENTIA. 



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iiig, however, those of Sigmoilon, tliough falling short of those of Hesperomys 

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

 peromys, and with llie general appearance of Sigmodon or even Mus. 



Furlher details of this interesting section, the 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 

 liloiisc" of (he 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. 



SktiH (Nos. -?JS!, ?J-li, 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 inich in zygomatic breadth. It has, how- 

 ever, other int(!resting peculiarities. Prominent among them i.s the sharp ridge 

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

 sh;irp the border of the orbit may be in the smaller Hesperomys, it does not 

 form this bead, which seems to characterize chielly larger forms, ivs we find it 

 in Mus dectimnnus, Sigmodon hispidus, &c. The ai'teorbilal 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 

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

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

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

 even l)eyond a point opj^site the anterior molars; the opposite is shown in 

 Ncotuma, where the foramina full far short of the molar series. The great 

 l)ackward production of the bony palate that Oryzomys shows does not occur 

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

 ate is about opi)i)site the last molar), and perhaps represents one extreme in 

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

 111!' middle molar, may l)c the other extreme. The little deep pit, or fossa, on 

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

 latter these pits are separated l)y a median lengthwise process, which is not 

 the case in Oryzomys, where the hind l)order of the l)ony palate is almost 

 prrfect/y straight crosswise. This i)ackward extension of the palate in Ory- 

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

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

 those of typical j\fiis of any Hesperomys of North America, if indeed they 



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