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104 



MONOOUAl'US OF NOllTU AMERICAN KODENTIA. 



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ajMjlogy ill tliis instance if melanophri/s turns out to be simply mexicanus. 

 At first, we referred our specimens to the latter with little hesitation; but, on 

 reviewing the subjt.'ct, they show so many differences from M. Ue Saussure'a 

 description that we cannot effect an identification. In general dimensions, 

 the two species, if sncli I hey are, seem nearly the same; but in tnczicanus, 

 the tail is only longer than the body alone, never even etjuuling the hea«l and 

 body together; while in. all our specimens the tail is considerably longer than 

 the head and body. This, however, taken alone, we should not consider a 

 specific difference, aflter our experience with Hcspcromys "myoitles" and 

 "boylii"'; but there are other differences. De Saussure speaks of grayish- 

 white under parts, with yellowish on the chin and breast; but in these speci- 

 mens the whole under parts are as uniformly pure white as in kucopus. lie 

 says the hind feet are brown ; in our animal, they are pure white, with a defi- 

 nite dusky area at the base of the metatarsus The tnil in his animal is 

 described as white below ; in ours, the bicoloration is not so evident as this 

 would imply. Finally, he makes no mention of the singularly conspicuous 

 black ring around the eye. All things considered, it is most probable that De 

 Saussure had in view an entirely different animal. 



The skull, as already stated, is extremely similar to that of Hesperomys 

 leucopus, the species thus falling legitimately in the restricted group, of which 

 leucopus is typical. The chief difference is the presence of a distinct bead on 

 the margin of the orbits, traceable nearly to the occiput. This, however, we 

 cannot consider as even subgeneric ; for, as elsewhere stated, it is only the 

 smallest and most delicate Hesperomys that are entirely devoid of this crest, 

 which makes its appearance with a very slight increase in the general size 

 and vigor of any s|)ecics. The dentition shows nothing peculiar. The pal- 

 ate, as in H. Ifucopiis, ends as a simple transverse shelf opjiosite the last 

 molar — a feature by which, as well as by its general slenderness, elongation 

 of rostrum, &c., it is distinguished from that of Oryzomys ; the latter agree- 

 ing in the development of a slight orbital bead. Skull No. fHt measures 

 1.20 in length by 0.G4 in breadth, thus obviously exceeding that of leucopus. 

 The coronoid process appears extremely short and slight, merely a little spicu- 

 lum ; the bulla; osseiE are noticeably small. 



Two female specimens show two pairs of inguinal mammae and one axil- 

 lary pair. A note of Professor Sumichrast's, accompanying one of these, 

 gives the following particulars: — "Nov. 30, 1862. Found among the leaves 





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