The molar series, (so far as can be judged from their broken condition), are much the same as in other species; except, perhaps, in the greater relative size of the posterior lobes, and in the more complete closing of the median valley by the junction of the two lobes externally. The ridge which curves downwards and inwards from the external point of the anterior lobe is unusually well developed, and curves across the entire crown. The cingulum is feebly shown; it appears on the anterior and posterior ends of the tooth (first molar) and for a very short distance on the outside of the tooth opposite the median valley; there is no trace of it on the inner side. _Measurements._ M. Length of entire molar series (about) ·080 Length of premolar series ·033 Length of true molar series (about) ·047 Fore-and-aft diameter of fang of canine ·008 Distance between canine alveoli ·009 Length of diastema ·115 Length of the first molar ·015 Breadth of the first molar ·010 Depth of ramus below last molar ·0245 Depth of ramus below last premolar ·0215 Greatest thickness of ramus ·017 Length of symphysis (about) ·030 Found at Henry's Fork. Hyrachyus -------- ? There are three premolars of the upper jaw which we cannot refer to any known species; but they are too imperfect to justify the formation of a new species for their reception. What appears to be a first premolar is inserted by a single broad fang; it is too much worn to show the characteristic enamel folding, and is peculiarly broad and short. The second premolar shows the usual foldings of _Hyrachyus_; save that a distinct valley enters the tooth from the posterior edge, running forward to the transverse crest, and inclosed by the curving backwards of the internal cone. The crown is nearly square; the basal ridge is scarcely marked at all. _Measurements._ M. Length of first premolar ·011 Breadth of first premolar ·016 Length of second premolar ·012 Breadth of second premolar ·014 HELALETES, Marsh. Am. Journ. Sc. and Arts, 1872, p. 218. Additional material enables us to give further characteristics of this genus, which may be thus defined: Mammals, allied to _Lophiodon_, having a third lobe to the last lower molar, short symphysis, and with the teeth of the lower jaw forming a continuous series. Astragalus has narrow, oblique condyles.