Chapter 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.

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._