30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 128 



much broader P* in H. quadricuspis, although as noted in the dis- 

 cussion of the preceding species, P4 in H. annectens may not be 

 normal. In the lower series P4 has a better-developed and rather in- 

 flated appearing metaconid. Also, the lower molars may be just a 

 little more crescentic. 



Hylomeryx quadricuspis is recorded as coming from a higher 

 horizon in the Uinta formation than H. annectens and may well be 

 descendent from it. 



Genus MESOMERYX Peterson, 1919 



Type. — Mesomeryx grangeri Peterson. 



Discussion. — Mesomeryx is a genus clearly distinct from the fore- 

 going, having comparatively simple, bunodont teeth. It is distinguished 

 primarily by the absence of both the mesostyle and hypocone of the 

 upper molars. The outer walls of the upper teeth are more like 

 Homacodon than are the corresponding teeth of Hylomeryx, as the 

 latter exhibits an incipient mesostyle. The outer portion of the upper 

 molars of Mesomeryx, and Homacodon as well, are surprisingly like 

 Hyopsodus. Mesomeryx differs from both Hylomeryx and Homa- 

 codon in lacking the hypocone in both M^ and M^. It should be 

 noted, however, that M^ in Meso^neryx exhibits a definite flexure of 

 the cingulum at the position of the hypocone, but this is in no way 

 comparable to the development of this cusp in either Homacodon 

 or Hylomeryx. In Bunomeryx the cusp, though feeble in M^, is 

 robust on M^ ; moreover, upper molars of Bunomeryx further differ 

 in the prominent development of a mesostyle. The extent to which 

 the protoconule is defined in Mesomeryx molars cannot be surely 

 determined owing to the degree of wear in the type and only known 

 specimen of the gentotype, M. grangeri. This portion of the upper 

 molars would appear to be developed as a protoloph, but in all prob- 

 ability the protoconule was distinct in less-advanced wear. The tri- 

 angular shape of the metaconule in advanced wear would suggest 

 that this cusp tended to be somewhat crescentic in Mesomeryx. 



The upper premolars in Mesomeryx appear to be unspecialized, 

 with P* rather like that in Bunomeryx. P', however, is somewhat 

 shortened anteroposteriorly, particularly in the anterior portion, giving 

 the deuterocone the appearance of having a more forward position on 

 the tooth. 



The lower teeth of Mesomeryx are not known. 



Mesomeryx appears to be the only Uintan artiodactyl known which 

 possesses a combination of characters suggesting an ancestral posi- 



