FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 145 



3iiore advanced in the reduction of the paraconid, but are much 

 younger. The broad third lobe of M3 and double hypoconulid also 

 appear in Necrolemur and Microchoerus , but not in Ahsarokius. Among 

 American tarsioids only Washakius has a similar talonid on M3, and 

 its trigonids are quite different. 



The characteristic posterointernal upper molar expansion and basin- 

 ing of Paromomys are suggested in many later tarsioids, such as 

 Jibsarokius, Hemiacodon, Shoshonius, Tetonius, and, in Europe, Nanno- 

 pithex (most marked in Ahsarokius and Nannopithex) , but in all these 

 the structure is much less marked. In some cases (e. g., Tetonius) it 

 is so slight that it is noted only by special search with Paromomys in 

 mind, and in all the instances mentioned there is the characteristic 

 distinction that a posterior cingulum passes internally beyond the 

 limits of the incipient (or vestigial) basin and tends to form a hypocone 

 at its inner end. This could be a specialization from the Paromomys 

 ^condition, but the difference is clear and there is no evidence of cer- 

 tainly intermediate stages. In the European Necrolemur (and its 

 highly modified ally, Microchoerus), however, the hypocone is on the 

 Tim of a basin much like that of Paromomys, and structural ancestry 

 as regards this character is quite possible but hypothetical. 



In summary comparison with the acknowledged tarsioids, there are 

 Tcsemblances throughout and every separate structure of the Fort 

 Union genera is approached in some later genus. The fundamental 

 rsimilarity is most clear in Palenochtha, but even in tliis most general- 

 ized type the anterior dentition is too specialized for ancestral rela- 

 tionship to any known later genera but Tetonius, Necrolemur, Micro- 

 -choerus, and (still more doubtfully) Ahsarokius, and in these cases the 

 minor morphological differences are also marked and annectant forms 

 unknown. Paromomys and Palaechthon also resemble various later 

 -genera, but in most cases crossing specializations make any approach 

 to direct phyletic connection impossible. Ahsarokius is, on the whole, 

 the most similar American form, but in several respects it is apparently 

 less specialized ; for instance, in the simpler heel of M3, probably less 

 -enlarged incisor, and smaller protocone of P* (which may, however, be 

 secondary), despite its younger age. 



The European genus Necrolemur ^^ compares more closely with 

 Paromomys than does any known tarsioid to the extent that it exhibits 

 all the principal speciahzed characters of Paromomys and that while 

 it has numerous additional specialization of its own, no crossing special- 

 zation is involved. Its dental formula is probably frijj. Stehlin 

 (1916) has placed this upper formula beyond any serious question. 

 He gives the lower formula as ^yixi; but the evidence is very uncon- 

 vincmg. A priori it is highly improbable that an enlarged median 



"The following remarks apply equally to Microchoerus, excppt that the latter is much more highly special- 

 ized in the dentition. If Necrolemur comes from Paromomys, then, ipso facto, Microchoerus does also but 

 has evolved more rapidly. 



