84 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



distinctive and resembles that in Helohyus and Antiacodon as well as 

 Homacodon. A difference, however, perhaps more noticeable with 

 reference to Antiacodon and Homacodon than to Helohyus, lies in 

 the higher, more inflated appearance of the deuterocone and somewhat 

 less transverse width of the tooth. 



The upper molars, as exhibited in U.S.N.M. No. 22248 and U. of 

 Wyo. No. 1779, have a rectangular appearance, rather distinctive in 

 comparison with Diacodexis. The protocone, though well developed, 

 is not so large and inflated as in Diacodexis and the accessory cusps, 

 the protoconule and metaconule, are larger and more conical than in 

 that genus. Particularly noticeable is the strong development of the 

 cingulum anterointernally and posterointernally, and weakly or not at 

 all across the protocone, giving the talon a square appearance. Antero- 

 internally, the nearly right-angle flexure of the cingulum shows little 

 or no evidence of a cuspule. Posterointernally, the cingulum rises 

 distinctly to form a low, well-separated hypocone with a slight crest 

 extending away from the cingulum toward the metaconule on M^, but 

 toward the protocone on M^. 



The upper molars, believed to be of Hexacodus pelodes, are seen 

 to resemble much more closely those of Microsus and Homacodon 

 than they do those of contemporary Diacodexis ; although the hypo- 

 cone is much less developed and the accessory cusps have better- 

 defined crests than in the Bridger genera. Homacodon differs from 

 Microsus essentially in more inflated cusps and shows development 

 of the cingulum lingually in upper molars. In the cf . Hexacodus speci- 

 mens the crest from the anterior portion of the metaconule to the 

 anterointernal part of the metacone and the posterior crest from the 

 metaconule to the cingulum are more pronounced. In A.M. No. 

 12146 of Microsus cuspidatus this posterior crest is absent, but in 

 A.M. No. 12696 it is weakly developed. A.M. No. 12696 of M. 

 cuspidatus resembles cf. H. pelodes in almost every way except in 

 greater development and more conical form of hypocone and gener- 

 ally more subdued crests. It should be noted, moreover, that while the 

 hypocone of cf. H. pelodes rises from the cingulum, in certain speci- 

 mens of Microsus, though not all, the cingulum has become developed 

 part way around the hypocone. Furthermore, the anterointernal angle 

 of M2 in cf. H. pelodes is a little less rounded than in Microsus and 

 both molars show a slight mesostyle or rise in the cingulum more sug- 

 gestive of Antiacodon. 



In Antiacodon, as represented by A.M. No. 12043, the cusps of the 

 upper molars are very conical and the hypocone in M3, at the lingual 



