FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 159 



Diagnosis. — Dental formula probably H^^. Anterior lower denti- 

 tion shorter than in Paromomys or Palaechthon and apparently with 

 one tooth absent, probably P2. P4 of about the same length relative 

 to Ml as in Palaechthon alticuspis but relatively higher, with no sign of 

 the metaconid and only vague rudiment of the paraconid. M1-2 

 similar to those of Palaechthon, but M3 with smaller third lobe and 

 single hypoconulid. Upper molars similar to those of Paromomys 

 and Palaechthon but very slender, transverse, and more triangular. 

 Posterointernal expansion weak. Inner base not bilobed. M^ 

 shorter relative to M^. 



Discussion. — In describing Palaechthon minor, Gidley (1923, p. 8) 

 said: "Most of the differences noted above suggest for the species just 

 described a slightly different line, or direction, of development than is 

 indicated in P. alticuspis. It is possible, therefore, that more complete 

 materials may prove that these two species do not form a natural 

 generic group." Although more complete materials are still lacking, 

 I do not see how this species can be placed in Palaechthon. The 

 further reduction of the anterior teeth, the absence of a metaconid and 

 of a distinct paraconid on P4 (not explicitly mentioned by Gidley), 

 and the single hypoconulid on M3 are just such differences as are used 

 to distinguish genera among all early primates. While it is true, as 

 Gidley notes, that the upper molars differ less from Palaechthon 

 alticuspis than do the lowers, still the differences are rather more 

 marked than are those between the latter species and Paromomys 

 depressidens . Unfortunately, the anterior upper teeth, which often 

 show more marked generic characters, are unknown, but I think there 

 can be no doubt that the genus is distinct. 



PALENOCHTHA MINOR (Gidley) 



Figure 33; Plate 10, Figure 1 

 Palaechthon minor Gidley, 1923, p. 7. 



T^/pe.— U.S.N.M. no. 9639, right lower jaw wdth P4-M3 and anterior 

 alveoli. Collected by A. C. Silberling. 



Horizon and locality. — All known specimens from Gidley Quarry, 

 Fort Union, Middle Paleocene horizon, Crazy Mountain Field, Mont. 



Diagnosis. — Sole known species of the genus as described above. 

 See description below and measurements in table 35. 



Discussion. — Teeth anterior to P4 are represented only by alveoli 

 in the type and in no. 9631. Both show an alveolus for a large, some- 

 what compressed, procumbent incisor, followed by a smaller, more 

 erect alveolus, evidently for a canine. Between this and P4 there 

 appears to be only a small double alveolus, or two very small alveoli 

 confluent at their mouths. It is higlily probable that this lodged one 

 tooth, P3, and in this event P1-2 must have been missing. There 



