FORT UNION OF CRAZY MOUNTAIIST FIELD, MONT. 229 



Ellipsodon, but the other characters of the dentition ahnost exclude 

 the possibihty of special relationship. 



Protoselene is a more sharply defined genus than any of the others 

 here considered. It has the general characters of a prmiitive hyopso- 

 dontid but is evidently becoming specialized throughout the dentition 

 in a way hardly suggested by any other genus. 



Hyopsodus is not inserted in the above key, because it is not known 

 in the Paleocene and because it is so distinctive that it can be recog- 

 nized at a glance, and confusion with the Paleocene genera is impos- 

 sible. This distinctive character, however, is entirely in features 

 demonstrably progressive, and anyone who studies the whole structure 

 of Hyopsodus, particularly with reference to the evolution that 

 occurred within that genus, can hardly fail to endorse Matthew's 

 conclusion that it is an ally of the Paleocene forms here discussed. 

 The genera discovered since Matthew's work still more strongly 

 substantiate the reality of this relationship, for it may now be said 

 that Hyopsodus has no known structural character not clearly 

 developed or adumbrated in the Paleocene hyopsodontids." 



Hyopsodus most nearly resembles group D of the foregoing key, 

 and m a general way this group has every essential requirement for 

 the structural ancestry of the Eocene genus. Its most exact resem- 

 blance in details appears to be with Haplaletes. The sequence 

 Oxyacodon-Haplaletes -Hyopsodus is, as far as it is known, one in 

 which no difficulty opposes its acceptance as a structural phylum. 

 There are no "crossing speciahzations", and all characters seem to 

 be modified uniformly and in one direction in accord with the relative 

 ages of the genera. At the same time it is, of course, apparent that 

 the data are inadequate to prove that this is an exact genetic 

 phylum, and, as in most cases, the probabilities are very much against 

 our having in collections the exact members of the true line of descent. 



There remams for discussion only Phenacodaptes Jepsen, 1930. 

 This has not been inserted in the key because its afiinities with 

 the other genera are not definitely established, and Dr. Jepsen has 

 material that he has not yet described and that may give a better 

 basis for decision. From his published data, the genus appears to 

 me to enter into the Hyopsodontidae. It has characters strongly 

 suggestive of the dichobunid artiodactyls, but so has the whole 

 Paleocene group of hyopsodontids. It is, indeed, almost impossible 

 to frame a diagnosis, on dental characters alone, that will surely 

 distinguish hyopsodontids and artiodactyls, yet such skeletal parts as 

 are known show that they were quite distinct, at least in the lower 

 Eocene, and even the dentitions give a definite feeling, supported 



" Mile. Friant's recent reference of Hyopsodus to the Insectivora and strange discussion ot the derivation 

 of insectivore, especially erinaceoid, molar patterns can be quite ignored. She seems to te wholly unaware 

 of any of the evidence for the true affinities of Hyopsodus. 



