GEOMYID4A—GEOMYS. 611 
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. 
GrENnuS GEOMYS (emend. ex Ratf.). 
Mus, sp. Suaw, et al., l. c. infra. 
Cricetus, sp. DESM., et al., l. c. infra. 
Geomys, Rar., Am. Month. Mag. ii, 1817, 45.—Barrp, Mamm. N. A. 1857, 368.—CouEs, Proce. Phila. Acad. 
1575, 180 (monographie sketch); Powell’s Rep. Colorado R. 1875, 220 (monograph); Bull. 
U.S. Geol. Surv. 2d ser. no. ii, 1875, pp. 81 seqq. (cranial characters). 
Diplostoma, Ra¥., op. et loc. cit. ; 
Saccophorus, Kun, Beitr. 1820, 65. 
Pseudostoma, Say, Leng’s Exp. R. Mts. i, 1823, 406. 
Ascomys, Licut., 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. Bulls ossez 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. On 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 contractile movements as the receptacle may be suscep- 
tible of. But the connection between the folds of skin is so slight and loose that the ponch may, with 
little force, be turned completely inside ont, 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 are drawn up with special reference to antithesis with those of Thomomys, 
beyond, 
