FORT Ui^ION OF CRAZY MOUNTAIN FIELD, MONT. 143 



both the other genera. Paromomys and Palaechthon show crossing 

 speciahzations. The general molar structure of Palaechthon is 

 specialized more or less in the direction of Paromomys but is less aber- 

 rant, while its P4 is definitely more progressive. Paromomys shows 

 distinctly the most aberrant molar structure but has P4 still relatively 

 unprogressive. 



The only known primate of comparable age is Plesiolestes Jepsen, 

 from a Torre j on equivalent in the Fort Union of northern Wyoming. 

 Its age is not appreciably different from that of the Gidley Quarry spec- 

 imens, and the geographic locality is not very distant, all occurring in 

 the same widespread formation. Jepsen tentatively referred his 

 genus to the Plesiadapidae but noted (1930a, p. 506) liiat "there are 

 many structures on the two specimens which are not like those of other 

 Plesiadapids." He did not compare with Gidle3^'s previously pub- 

 lished genera, which Plesiolestes resembles in many ways. The 

 anterior alveoli show an enlarged semiprocumbent incisor and a 

 smaller, less procumbent canine, as in all three of Gidley's genera 

 here discussed, and also a moderate P2, as in Paromomys and Palaech- 

 thon. P3 is also closely similar, but relatively larger, being about as 

 high as P4, whereas in Gidley's genera it is lower. P4 closely resembles 

 that of Palaechthon, the only difference clear from the available data 

 being that in Plesiolestes the heel is wider and the paraconid and meta- 

 conid stronger, especially the latter. The molars are very similar to 

 those of Palaechthon and seem to me to show no difference of probably 

 generic value. 



While Plesiolestes may be provisionally accepted as valid, chiefly on 

 the basis of the more progressive P4, it is almost surely very closely 

 related to Palaechthon, and the distinction of the genera is not at pres- 

 ent wholly satisfactory. If, as is possible, Plesiolestes is somewhat 

 younger, it could well be a slightly modified and progressive descend- 

 ant of Palaechthon. Its diagnostic features, as against Palaechthon, 

 seem to me modifications away from as much as toward the plesiada- 

 pids. In any case it is surely closer to Palaechthon than is either genus 

 to any undoubted plesiadapid, and if a plesiadapid relationship exists 

 at all, Palaechthon is probably less removed from that line than is 

 Plesiolestes. 



Palenochtha, the least aberrant of the present genera as regards 

 comparison with an abstract protoprimate dentition, seems to resemble 

 the Eocene tarsioids (in the broadest sense) more than any other 

 known mammals. The specialization of the anterior lower teeth is 

 not exactly as in any later tarsioid but is within the apparent poten- 

 tiaUties of the group. Omomys, from the Bridger,^° has two enlarged 



«« The American lower Eocene species placed in Omomys are very doubtfully congeneric with the Bridget 

 genotype. As noted by Teilhard, the European lower Eocene specimens are very distinctive and might 

 be, in my opinion definitely are, representative of a different genus but one close to and perhaps structurally 

 ancestral to true Omomys. 



