FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 227 



B. F*4 cuspidate, more or less enlarged but not inflated. 



3. P4 without distinct paraconid, metaconid absent or rudi- 



mentary, talonid relatively wider, M^3 much to somewhat 

 reduced. 

 Ellipsodon (inaequidens*, prisons, leniuroides, acolytus, aquilonius) 



4. P4 with rudimentary paraconid, metaconid relatively large, 



talonid narrow, M^ relatively large Litaletes (disjvinctus*) 



//. Paraconids median to subinternal, not fusing with metaconids, 

 entoconlds distinct, molar talonids basined. 



C. Teeth more lophiodont, P4 bicrescentic, paraconids distinct, 



P^ with metacone, M'"- with strong mesostyle, M^3 large. 

 5 Protoselene (opisthacus*) 



D. Teeth more bunodont, paraconids reduced or vestigial, no 



mesostyles, M^3 generally somewhat reduced. 



a. P4 without distinct paraconid, but relatively elongate and 



trenchant, M3 large, with projecting hypoconulid, molar 

 cusps acute. 



6. P4 with rudimentary metaconid, molar paraconids rela- 



tively large Oxyacodon (apiculatus*, agapetillus, priscilla) 



7. P4 with distinct metaconid, with a pit between this and 



protoconid, molar paraconids reduced Litomylus (dissentaneus*) 



b. P4 with distinct small paraconid but relatively wide and 



heavy, M3 somewhat reduced, molars bunodont, generally 

 broad and low. 



8. P4 with smaller metaconid and talonid, talonid of M3 less 



elongate, upper molars more transverse and angulate, 

 outer cusps of P^"* more compressed. 



Litolestes (ignotus*, notissimus) 



9. Metaconid and talonid of P4 larger, M3 more elongate, P^~* 



with more conical outer cusps, upper molars rounded and 



less transverse Haplaletes (disceptatrix*) 



c. P4 elongate, with strong paraconid, M^3 reduced, prominent 



cingulum descending from protocone tip. 

 10 Haplo mylus (speirianus*) 



Like all keys, this is artificial, but it is based oq an extensive analysis 

 of the characters of all the species listed, and an effort has been made 

 to select characters that have clear taxonomic value and are probable 

 or possible indications of phyletic relationships. Thus it is believed 

 to be probable that the capital letters indicate four natural groups of 

 genera. The primary division, indicated by Roman numerals, also 

 appears to me (but with somewhat less probability) to be a natural 

 dichotomy of the whole group. 



Matthew has suggested that Choeroclaenus turgidunculus, which he 

 referred to Oxyacodon, might be ancestral to one or more species of 

 Ellipsodon. The resemblance is certainly close, and I believe a 

 relationship to exist. It seems to me, however, to be that indicated 

 above by group / and hence more general than he suggested and 

 inclusive of Alioclaenus, sensu stricto. In more exact phyletic relation- 

 ship, Choeroclaenus turgidunculus appears to be closer to Mioclaenus 

 turgidus The resemblance in the molars is as close as to Ellipsodon, 



