GEOMYID/E— GEOMYS. Gil 



The species of both are subject to a curious dichromatism, being sometimes 

 found in a certain melanotic condition, in which the pelage is dark plumbago- 

 colored, or even black, 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. 



Genus GEOMYS (emend, ex Raf.) . 



Hits, sp. Shaw, et ah, h c. infra. 



Cricetus. sp. Dksm., et ah, I. c. infra. 



Geomys, Rap., Aiu. Mouth. Mag. ii, 1817, 45. — Baird, Mamra. N. A. 1857, 363.— Coues, Proc. Phila. Acad. 



1875, 130 (monographic sketch); Powoll's Rep. Colorado R. 1875, 220 (monograph); Bull. 



U. S. Geol. Surv. 2d ser. no. ii, 1875, pp. 81 seqq. (cranial characters). 

 Diplostoma, Raf., op. et loc. cit. 

 Saccophorus, KrjHL, Beitr. 1820, 65. 

 rseudostoma, Say, Long's Exp. R. Mts. i, 1823, 406. 

 Ascomys, Licht., Abhand. Berl. Acad. 1822-23, 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. Bullae ossese 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 deep 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. 



cate texture. It is clothed with fine fur. Ou the side next the head, the ordinary fur of the parts 

 makes directly into the pouch ; on the outside, the fine fur continues to the brim, where it is met by the 

 ordinary external pelage. Between the two layers of skin rests a thin bed of muscular fibres (perhaps 

 a modified platysma myoides), serving for such coutractile movements as the receptacle may be suscep- 

 tible of. But the connection between the folds of skin is so slight and loose that the pouch may, with 

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

 full-sized pouch will admit three fingers as far as the first joint." — (Quoted from the original memoir.) 



"The characters aro drawn up with special reference to antithesis with those of Thomomys, 

 beyond. 



