52 



Measurements. M. 



Length of first molar * 12 



Breadth of first molar -OI55 



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 narroVer, 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 semicircle, 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, per- 



