This species is intermediate in size between _H. agrarius_ and _H. modestus_. As in the latter species; the basal ridge extends around the entire crown of the tooth, with a small interruption on the antero-external lobe. It is more pronounced, in front and less behind than in _H. imperialis_. The anterior conical buttress is not nearly so well developed as in _H. imperialis_; the valleys are wide, and the descending fold from the antero-external lobe is obscure, scarcely marked at all in the first. The transverse crests are strongly arched forwards, and the anterior is very much the longer. The peculiar feature of the species is the position of the postero-externa, lobe, which is pushed to the posterior aspect of the tooth; this is carried so far in the last molar that the two external lobes stand on nearly the same transverse line. The antero-external lobe is much the highest point in the tooth, and its the first molar is remarkably sharp. The enamel is considerably wrinkled, but not nearly so much as in _H. modestus_. _Measurements._ M. Length of first molar ·012 Breadth of first molar ·0155 Length of second molar ·015 Breadth of second molar ·017 Length of third molar ·0145 Breadth of third molar ·017 From Cottonwood Creek. Hyrachyus nanus, Leidy. Pr. Ac. Nat. Sc., 1872, 20. Henry's Fork. Hyrachyus agrarius, Leidy. Pr. Ac. Nat. Sc., 1871, 229. Henry's Fork. Hyrachyus eximius, Leidy. Pr. Ac. Nat. Sc., 1871, 229. Cottonwood Creek. Hyrachyus crassidens, _sp. nov._ Established on the left and a portion of the right ramus mandibuli, containing the first molar. The jaw presents some differences from the form common in Hyrachyus. The alveolus is more curved, especially near the ascending portion of the ramus; the jaw is narrower, and is especially contracted near the incisive alveolus. The lower margin of the ramus is but slightly curved. The ramus itself thickens regularly from before backwards, and is thickest at the last molar; beyond which it becomes thinner again. The depth of the jaw is comparatively small, and the teeth have a forward inclination; the mental foramina are not so large as in other species of _Hyrachyus_, and are four in number. The incisors are small, arranged in a semi-circle, and have very much compressed fangs, with somewhat procumbent crowns; the canines are small, and form a continuous series with the incisors. There is a diastema between the canine and first premolar; but one which is shorter than in known species of the genus, and which does not indicate the usual curve of the upper border of the ramus.