CEOMYID4—THOMOMYS. 621 
flange-like expansion of the malar anteriorly, where, at the point of greatest 
width, the bone is 0.35 in breadth; the process extends obliquely downward 
and outward, with rounded contour, Nothing equalling this special expansion 
of the malar is seen in any of the United States species, skulls of whick 
have been examined, though the bone is usually thicker in the corresponding 
portion of its course than elsewhere ; and, in G. castanops particularly, is some- 
what laminar at the corresponding point. The skull as a whole is “rugged”, with 
the several ridges and angles highly developed; the jaw is specially massive. 
There seems to be a peculiarity in the “set” of the lower incisors, apparently 
determined by their great protrusion. In G. dursarius, holding the jaw hori- 
zontal, 7. e. with the crowns of the molars on a level, the incisor-tips still 
incline strongly obliquely forward. In the present species, when the jaw is 
similarly viewed, the ends of the incisors are vertical, so great is their curva- 
ture. Various peculiarities of the skull might be noted, but they may be 
considered covered by the statement of its great massiveness, roughness, and 
angularity. The pattern of the molar crowns seems to be much the same. 
Genus THOMOMYS, Maxim. 
Diplosioma, Ricuarps, Fn. Bor.-Am. i, 1829, 206 (nec Raf.), 
Oryctomys, pt. ED. & GERV , Mag. Zool. vi, 1836, 23. A 
Thomomys, MaxiM., N. Act. Acad. Cas. Leop. xix, 1839, 383.—Ba1rpD, M.N. A. 1857, 388.— Cougs, Proc. Phila. 
Acad. 1875, 134 (monographie sketch).—Covuks, Powell’s Rep. Colorado R. 1875, 243 (nono- 
graph).—Cousgs, Bull. U.S. Geol. Surv. 2d ser. no, ii, 1575, pp. 81 seqq. (cranial characters), 
Tomomys, Branpt, Beit. Kennt. Siiug. Russl. 1855, 187. 
(In addition to the foregoing, all the synonyms of Geomys, q. v., have been applied to this genus.) 
GENERIC cHARS.—Superior incisors without median sulcus, but with a 
fine marginal groove (sometimes obsolete). Crowns of intermediate molars 
acute-edged exteriorly. Root of inferior incisors causing a protuberance on 
outside of base of condylar ramus nearly as high as condyle itself; end of 
mandible thus singularly three-pronged. Zygomata regularly convex out- 
ward, with a sweeping curve; their breadth across posteriorly decidedly 
greater than the intermastoid diameter of the skull. Parietals ridged exter- 
nally near the squamo-parietal suture. Interparietal rather pentagonal. 
Nasals widening uniformly from behind forward. Superficies of mastoid 
bone restricted to less than a fourth of the occipital surface on each side. 
Bulle osseze more inflated, quite obtuse anteriorly. Basi-occipital, in the mid- 
dle, much narrower than the bulla at the same point. <A pair of slight pits 
on the palate behind, not extending beyond the ultimate molars. External 
