NO. 7 TERTIARY APHELISCUS AND PHENACODAPTES GAZIN 3 



the Silver Coulee or Tiffanian horizon of the Polecat Bench forma- 

 tion in the Big Horn Basin. The possibility of a relationship to artio- 

 dactyls was tentatively suggested by Jepsen (1930) because of resem- 

 blances noted to such genera as Diacodexis and Bunophorus, evident 

 in certain features of the molars. Simpson, however, in his classifica- 

 tion of the mammals (1945) cited Phenacodaptes as a condylarth 

 under ?Mioclaeninae incertae sedis. 



COMPARISON OF APHELISCUS AND PHENACODAPTES 



A lower jaw of Apheliscus, referred to A. insidiosus, in the Na- 

 tional Museum collection (No. 19 162) from the Gray Bull beds in the 

 Big Horn Basin, exhibiting P2-M x inclusive (see pi. I, fig. 1), shows 

 that the form and relative proportions of the lower premolars are 

 strikingly like those in Phenacodaptes (see pi. 1, figs. 3, 4). The rela- 

 tively small size of P 2 and particularly of P 3 in comparison with P 4 

 is quite alike in the two. P 4 is a little more slender in Apheliscus and 

 the distinctive talonid seen in this tooth of Phenacodaptes is more 

 sectorial and essentially better developed or exaggerated in Apheliscus. 

 Both have a strongly developed primary cusp and only slight evidence 

 of a paraconid. There is no metaconid on P 4 in the known material of 

 Apheliscus. It is usually absent, but may be weakly developed in some 

 specimens of Phenacodaptes. The lower molars differ noticeably in 

 the anteroposteriorly shorter trigonid and more elongate talonid 

 in Apheliscus (see pi. 1, fig. 2) ; moreover, they are relatively more 

 slender than in Phenacodaptes. There is, nevertheless, a rather 

 marked similarity in many details, particularly in form of the cusps 

 and crest surrounding the talonid basin, and in the shape of the basin. 

 The compressed trigonid of Apheliscus is rather less like that in 

 Phenacodaptes, although the paraconid is absent or very much re- 

 duced on the posterior two molars of both forms. In M x of Phena- 

 codaptes, however, this cusp is moderately well defined as an anterior 

 crest from the protoconid, whereas in Apheliscus only a slight median 

 cuspule remains. 



The upper cheek teeth of Apheliscus insidiosus (see pi. 2, fig. 1) 

 may appear a little less like those of Phenacodaptes (see pi. 2, fig. 2) 

 than perhaps do the lower teeth, although both exhibit the compara- 

 tively small and subequal second and third upper premolars and en- 

 larged fourth. The more noticeable differences between the two in 

 upper teeth include less development of the cingulum, particularly 

 on P 4 , and the transversely narrower molars of Apheliscus. More- 

 over, the hypocone, though distinctive on Mi and M 2 of Phenaco- 

 daptes, is weak or absent in Gray Bull Apheliscus. It is important to 



