156 BULLETIN 169, UNITED STATES NATIONAL MUSEUM 



marked specific distinction suggest that complete, unworn dentitions 

 might prove generic distinction, but the material actually in hand does 

 not warrant such a conclusion. 



As the samples are small, measurements of all known specimens are 

 given in table 32 (see also fig. 3). 



Genus and Species Undetermined 

 Cf. PAHOMOMYS 



In 1932 Silberling and I found at Loc. 13 a single left AP, apparently 

 representing an otherwise unknown primate. It resembles Plesiadapis 

 anceps but not very exactly and is still less like other species of that 

 genus. It is probably too small to belong to Plesiadapis rex, which 

 occurs at the same locality. It resembles Paromomys maturus more 

 closely than any other species with which comparison has been made, 

 but it is more transverse, has the inner face even longer and more 

 sloping, and has the anterointernal, not posterointernal, basal part 

 definitely more projecting. It also resembles the most primitive 

 species of Pelycodus but could not belong in that genus. Such an 

 isolated tooth is inadequate for generic, or even for certain family 

 identification, but its presence seems worth recording. 



Genus PALAECHTHON Gidley 



Palaechthon Gidley, 1923, p. 6. 



Type. — P. alticuspis Gidley. 



Distribution. — Middle Paleocene, Fort Union, Montana. 



Diagnosis. — Dental formula probably \' l 3 | . Lower incisor and 

 canine about as in Paromomys, or possibly incisor slightly larger and 

 canine slightly smaller. Roots of P2 less divergent or incompletely 

 divided. P4 larger relative to Mi, taionid as in Paromomys, but 

 trigonid with distinct, subequal paraconid and metaconid. Alolar 

 trigonids very similar to Paromomys but on M2_3 paraconid slightly 

 more distinct, lower on the crown, and less marginal. M3 wdth double 

 hypoconulid, but third lobe less strong than in Paromomys. Trigo- 

 nids more elevated. Upper molars somewhat more transverse than 

 in Paromomys maturus, internal bases less distinctly bilobed, and 

 posterointernal expansion less marked. 



PALAECHTHON ALTICUSPIS Gidley 



Plate 7, Figure 1 ; Plate 9, Figures 5, 6 



Palaechthon alticuspis Gidley, 1923, p. G. 



Type.— U.S. N.M. no. 9532, right lower jaw with P2-M2. Collected 

 by A. C. Silberling. 



