228 BULLETIN 1(3 9, UNITED STATES NATIONAL MUSEUM 



and the premolars are practically those of Miodaenus in miniature and 

 unlike those of Ellipsodon. Furthermore, the rather poorly known 

 Ellipsodon priscus carries that genus, or something very like it and 

 probably closely related, back into the Lower Paleocene, contempo- 

 raneous with Choeroclaenus . 



The group of species referred to Ellipsodon is rather heterogeneous, 

 as discussed under that genus. The presence of so many varied 

 species shows that several divergent minor phyla are present, but all 

 appear to be rather closely allied. Ellipsodon priscus represents a 

 possible an estral tj^pe of structure, without being clearly allied to 

 any particular one of the Middle Paleocene species. The other species 

 are all approximately contemporaneous and so represent a spreading 

 out of the group without permitting the discernment of any special 

 lines of descent. 



Oxyacodon represents the second major group in the Lower Paleo- 

 cene. Its distinctive characters are almost entirely primitive and it 

 affords a structural ancestry for its general group, D. It is improbable 

 that the ancestry of Protoselene would enter into Oxyacodon, and the 

 case of Haplomylus is also dubious. Upper teeth of Oxyacodon are 

 unlaiown, and might considerably modify the present conception of 

 the genus. 



Litomylus very closely resembles Oxyacodon but is in at least two 

 respects, molarization of P4 and reduction of molar paraconids, a more 

 advanced form. As far as the scanty data go, it could be a relatively 

 unprogressive descendant of Oxyacodon. 



Haplaletes and Litolestes, both possible structural derivatives of 

 Oxyacodon, are successive, Middle and Upper Paleocene, respectively, 

 and appear to be close relatives, but they cannot be along exactly the 

 same line of descent, at least in the known species. Litolestes, the 

 later genus, is probably more specialized in the reduction of M^3 and 

 perhaps in the more transverse upper molars, compression of P^~^, 

 and some other details, but its premolars seem to be slightly but dis- 

 tinctly less progressive than in Haplaletes. (Its known species are 

 also somewhat smaller than Haplaletes disceptatrix.) 



Haplomylus appears at the end of the Paleoc6i_3 and runs into 

 the lower Eocene. It is clearly a member of this general group, but 

 none of the older genera is enough like it to suggest any very close 

 structural ancestry. Its general premolar and molar structure, 

 although somewhat more advanced as would be expected, is of the 

 type of group D of the foregoing key and is such impelling evidence 

 of relationship that the genus has been classed with that group. 

 At the same time some important details are not foreshadowed in 

 any of the other genera. The most striking point, the development 

 of the posterointernal part of the upper molars as a broad cingulum 

 sweeping down from the protocone, curiously reminiscent of some 

 of the ancient primates, is approached (but not very closely) in 



