G E0MYIDJ3— THOMOMYS. 021 



flange-like expansion of 1 lie malar anteriorly, where, af the point of greatest 

 width, the bone is 0.35 in breadth; (lie 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 which 

 have been examined, though the bone is usually thicker in Ihe 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. bursarius, holding the jaw hori- 

 zontal, i. 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. 



IHplos'oma, Richards, Fu. Bor.-Atn. i, 1829, 20G {nee Raf.). 

 Oryctomys, pt. Eyd. & Gehv , Mag. Zool. vi, 18156, 23. 



Tliomomys, Maxim., N. Act. Acad. Cacs. Leop. xix, 1839, 383.— Baird, M. N. A. 1857, 388.— Coues, Proc. Phila. 

 Acad. 1875, 134 (monographic sketch). — Coues, Powell's Rep. Colorado R. 1K75, 243 (mono- 

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

 Tomomys, Brandt, Beit. Kennt. Siiug. Russl. 1855, 187. 



(In addition to the foregoing, all the synonyms of Genmys, 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. 

 Bulla? osseas 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 



