NO. I LOWER EOCENE MAMMALIAN FAUNAS — GAZIN 47 



very low crowned. Cusps low and conical. Talonid basin large but 

 very shallow and trigonid basin scarcely discernable. Except for 

 posterolophid, the transverse lophs are comparatively weak and ill 

 defined. The mesoconid is clearly defined and there is a distinct 

 hypoconulid. 



New material of Knightomys senior includes a pair of lower jaws 

 (U.S.N.M. No. 22415) with P4-M2 on right and M2-M3 and a part 

 of Ml on the left ramus. The upper teeth are not known. P4 is narrow 

 across the trigonid but was evidently bicuspid. The talonid of this 

 tooth has a shallow basin and the entoconid is simple and conical. A 

 slight ectolophid and better-defined posterolophid join the hypoconid of 

 P4 in the type, U.S.N.M. No. 19308, but these are scarcely evident in 

 No. 22415. There is no mesoconid on P4. The molar trigonids are 

 low and anteroposteriorly short with only the slightest evidence of 

 an anterolophid or even a cingulum, and this only on Mi. Although 

 the teeth in neither of the two preserved specimens are unworn, the 

 saddle between the protoconid and metaconid, except for a slight 

 depression in Mi, shows no evidence of a trigonid basin. The ecto- 

 lophid is very weak but the conical mesoconid is distinct. This cusp 

 does not have a crest extending lingually. The entoconid is nearly 

 conical but does have a weak crest extending a short distance into 

 the basin toward the hypoconid, but falls far short of reaching this 

 cusp. The hypoconid sends a more distinctive posterolophid postero- 

 internally, clearly separated from the entoconid and its weak spur. 

 The posterolophid is moderately worn on the specimens at hand but 

 there is clear evidence of a distinct hypoconulid. 



Material of Knightomys senior seems without doubt to be closer 

 to Tillomys? parvidens than to any other sciuravid. However, 

 comparison with M2 in U.S.N.M. No. 17701 of T.f senex from the 

 lower Bridger and with the figure of the type of T.f parvidens, 

 Y.P.M. No. 13350, given by R. W. Wilson (1938, fig. 12), reveals 

 that K. senior has lower-cusped teeth with less development of trans- 

 verse lophids. M2 is only slightly smaller than in the Bridger material 

 at hand, but the trigonid appears less elevated and shorter antero- 

 posteriorly. Also, the mesoconid is more nearly conical and the pos- 

 terolophid does not project so far lingually. The teeth of Tillomys 

 senex, figured by Wilson (1938, fig. 10), seem more remote, as the 

 weak spur from the entoconid in K. senior does not reach the hypo- 

 conid or the posterolophid as in the American Museum specimen. It 

 should be noted, however, that this T. senex specimen shows more 

 advanced wear. As observed by Wilson, Tillomys f parvidens, and I 



