GEOMYID.E— OEOMYS. 



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Tlio species of hotli are subject to a ciirioiis dichroniatism, being soiiiotimcs 

 found in a certain melanotic condition, in which liic pelage is dark pluml)ag()- 

 colored, or even l)lack, like anthracite. This appears too frequent not to be 

 something more than the purely fortuitous melanism liable to occur as an indi- 

 vidual peculiarity in any animal ; and yet this state has not been traced to 



any special conditions of age, sex, or season. 



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Genus GEOMYS (eniewrf. ea; Raf.). 



Mtn, ip. Shaw, et al, I c. infrd. 



Criceiut, tp. Drsm., et al, I. o. infrA. • > . 



Oeomyi, Raf., Am. Mouth. Mag. ii, 1817, 45.— Baird, Hnmm. N. A. 1857, 30d.— CouES, Proc. Phlln. Acul. 



Iri75, 130 (moiioKraiihio Bketcb) ; Powell's Rep. Colorado R, 187r>, !»0 (monograpli) ; Bull. 



U. S. Geol. Surv. 3d oer. no. ii, 1876, pp. 81 seqq. (craniiU ohiirnotorB}. 

 Diplmtoma, Rap., op. et locoit. 



Saccophorui, Kvni., Ovitr. l»JO, Gb. ■>■• ' 



PKudottomo., Sat, Long's Exp. R. Mta. i, 1823, AOr,. 

 Atcomyt, Licnr., Ablinnd. Berl. Acnd. 1822-2:1, 1825, 20. 



Generic characters.* — Superior incisors deeply channeled along the 

 middle, with or without a fine marginal groove. Crowns of intermediate molars 

 truly elliptical. Root of inferior incisor but little protuberant on outside of base 

 of condylar ramus ; end of mandible thus only two-pronged, with a knob 

 between. Zygomata widest across anteriorly, thence contracting; the width 

 behind little, if any, greater than the intermastoid diameter of the skull. 

 Parietals ridged along their line of union with each other. Interparietal tri- 

 angular. Nasals approximately parallel-edged part way, then suddenly widen- 

 ing. Superficies of mastoid bone occupying nearly half the occipital surface 

 of the skull on each side. BuUje osseae less inflated, quite acute anteriorly. 

 Basi-occipital, in the middle, about as broad as the width of the bulla at the 

 same point. A pair of broad dee;.' pits on the palate behind, extending for- 

 ward to opposite the penultimate molars. External ears usually a mere rim of 

 integument around the auditory orifice. Fore claws enormously developed. 



Containing the largest species of the genus. Geographical distribution 

 from portions of British America to Central America, east of the Rocky 

 Mountains, but not the Eastern and Middle States. 



cnle texture. It is clothed with fine fnr. On the side next the bead, the ordinary fur of the parts 

 nmkes directly into the pouch ; on the outside, the floe tar continues to the brim, where it is met by tlie 

 ortliuury external {wlage. Between the two layers of skin rests a ihin bed of muscular flores (pcrliaps 

 a uiodifled pliityHma myoides), serving for such contractile mnvemeuts as the receptacle may be snscep- 

 tibia of. But the connection between the folds of skiu is so slight and louse tbit tlie pouch may, witli 

 liltio force, be tamed completely inside out, though it does not appear that this ever occurs in life. A 

 rull-si7.cd pouch will admit thiee Angers as far as the first Joint." — (Quoted from the original metHOir.) 



'The characters are drawn up with special reference to antithesis with those of Thomomyi, 

 beyond. 



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