148 BULLETIN 169, UNITED STATES NATIONAL MUSEUM 



indeed, very close, and as far as the posterior teeth are concerned the 

 differences involve only a few minor details of apparent generic or 

 lesser value. The anterior teeth, however, are very different and are 

 much more specialized in the earlier genus. Pelycodus has the formula 

 ^xza as against yj:^^ in Paromomys and Palaechthon, its incisors 

 are small and its canine large. Furthermore, the Paromomys-like 

 molar characters of Pelycodus are not seen in Adapis, but the Notharc- 

 tinae, to which Pelycodus belongs, and the Adapinae show a funda- 

 mental resemblance in skull and skeletal structure, which leads 

 (Gregory, 1920, and elsewhere) to their association in one family. 

 And this general structure is very unlike that of the supposedly 

 tarsioid genera, such as Tetonius or Necrolemur, with which there is. 

 equal or greater reason for supposing Paromomys to be related. 



To propose as a tentative solution of this extraordinarily intricate- 

 problem that Paromomys is a derivative of a tarsioid-notharctine- 

 ancestry is not fully satisfactory, for a corollary would be that the- 

 Notharctinae were descended from a different tarsioid or pro to tarsioid 

 ancestry from the Adapinae, one with more Paromomys-like molars- 

 and this is wholly unsatisfactory on the basis of the mutual relations 

 of Notharctinae and Adapinae as inferred from their own much, 

 better known structures. It is much more probable either that the 

 marked resemblance in the molars of Paromomys and Pelycodus i& 

 wholly convergent or that Paromomys is really a divergent offshoot 

 of the earliest notharctine ancestry toward wiiich various tarsioids- 

 have converged in one way or another. The second alternative would 

 imply extreme antiquity of the Pelycodus pattern and the very remote- 

 separation of Notharctinae and Adapinae, to a degree that seem& 

 improbable. The very tarsioid, and not particularly Pelycodus-like^ 

 pattern of Palenochtha, which nevertheless seems almost surely to be- 

 a fairly close relative of Paromomys and Palaechthon, and the many 

 distinctly tarsioid characters of the latter genera, however electic in 

 their combination, also suggest that the former alternative is more- 

 probable, as Gidley concluded. I must confess, however, that I see- 

 no way of forming a really strong and reasonable opinion on this, 

 problem from the present evidence. 



Genus PAROMOMYS Gidley 



Paromomys Gidley, 1923, p. 3. 



Type. — P. maturus Gidley, 1923. 



Distribution. — Middle Paleocene, Fort Union, Mont. 



Diagnosis. — Dental formula probably tjM- Lower incisor enlarged ;>, 

 root extending beneath P2. Canine normal or slightly reduced^ 

 P2 present, 2-rooted. P4 not enlarged, paraconid and metaconid 

 very rudimentary or absent, trigonid apex slightly higher than Mr^ 

 iieel low, bicuspid. Molars with short trigonids, with closed, small^. 



