6 



The molars differ from those of the larger Hyrachyi, and resemble those 

 of the smaller, in the presence of a prominent ridge which descends on 

 the inner side of the principal (median) outer cusp, not quite reaching 

 the valley below. It wears into a prominent loop. The anterior cusp is 

 much less elevated than the median, and is separated from the latter by 

 a considerable ridge. The only cingulum on the molars is on the outer 

 side of the first ; enamel smooth. 



Measurements, No. 1. M. 



Length of five molars 0710 



" three posterior molars 0470 



" last molar 0159 



Width " 0200 



" penultimate molar 0210 



Length " 0168 



In the more perfect specimen, all of the molars have two transverse 

 crests except the P. M. 1. The lower molars possess strong anterior pro- 

 longations of their posterior crests ; the 3d and 4th premolars have one 

 elevated transverse crest near their middle, and the second is much com- 

 pressed. The first I cannot find. Symphysis rather short for the genus. 

 Measurements No. 2. M. 



Length superior molar series 0.085 



" true molars 046 



" penultimate .015 



Width " 019 



Length inferior molar series 078 



" " true molars 0475 



" penultimate 0170 



Width " .0110 



" last premolar 0080 



Length " " 0120 



Depth ramus at last premolar 0235 



Length diastema 0190 



' ' of bases of three incisors 0180 



Hyrachyus pbinceps. Marsh. 

 South Bitter Creek. 



Hybachyus eximius. Leidy. 

 Cottonwood Creek, common. 



Hybachyus agbarius. Leidy. Lophiodon bairdianus. Marsh. 

 Common everywhere. 



Hybachyus boops. Marsh. 

 Bitter Creek, Black's Fork. 



Hyrachyus nanus. Marsh. 

 Cottonwood Creek. 



Published, March 8th, 1873. 



