12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 128 



ing possibility in the combination of characters, Mesomeryx early lost 

 the hypocone but did not develop a mesostyle. In Mesomeryx, like 

 Hylomeryx, however, a suggested tendency is noted toward the forma- 

 tion of a protoloph at the expense of the protoconule. 



As will be discussed in somewhat greater detail in the systematic 

 portion of this paper, there would appear to be some justification for 

 regarding Simimeryx and the hypertragulids as derived from the 

 homacodonts at some point near Mesomeryx. There is, however, a 

 peculiar resemblance, which probably should not be disregarded, of 

 upper and lower molars of Simimeryx to the agriochoerids. It is 

 perhaps a similar step in the transition of each from the bunodont to 

 the more-selenodont pattern, so that the change from a bunodont 

 homacodont, such as Mesomeryx to Hypertragulus, included the 

 Simimeryx stage ; possibly equivalent to Protoreodon petersoni in its 

 suspected relationship to the Oligocene merycoidodonts. Attention is 

 likewise called to the very divergent origins indicated for hyper- 

 tragulids and leptomerycids. Regardless of an apparent modification 

 toward a similar mode of existence, they show striking dissimilarities 

 in details of dentition. Retaining these in the same family, as pointed 

 out elsewhere, is untenable in view of the polyphyletic origin indicated. 



A third subfamily of dichobunids, the Helohyinae, is represented 

 by what appears to be a nearly continuous sequence from Helohyus 

 to Achaenodon. The surprising increase in size is foreshadowed in 

 Helohyus lentus of Bridger D, and the interval between Helohyus and 

 Achaenodon is nicely bridged both in size and tooth characters by 

 Parahyus vagus, the type of which is almost certainly from the 

 Washakie beds. A simplicity of premolars is evident throughout, 

 possibly with the loss of one between Helohyus and Parahyus. The 

 bunodont character of the Helohyus molars is further emphasized 

 and simplified in Achaenodon with the reduction of the protoconule 

 and loss of the vestige of a hypocone in upper teeth of Achaenodon 

 and with the loss of the paraconid (vestigial in Mi of Parahyus) and 

 hypoconulid (except Mg) in the lower molars. Helohyus would ap- 

 pear to be most like Homacodon of the various homacodonts in the 

 simplicity of the premolars, but is unlike Homacodon in the near ab- 

 sence of a hypocone in the upper molars and the presence of a para- 

 conid in the lowers. The Helohyinae might well have had roots con- 

 verging with those of Homacodon before loss of the paraconid and 

 the development of so prominent a hypocone in the latter. The early 

 helohyids are perhaps structurally intermediate between diacodexids 

 and homacodonts. 



The entelodonts were not, of course, derived from Achaenodon, 



