78 



The penultimate lower premolar (Fig. 8) is a reduced form of the one 

 behind, with the internal median conical lobe obsolete. 



The lower jaw has its two rami co-ossified at the symphysis. It is thick 

 and rounded at the base, which is convex fore and aft beneath the molar 

 series. The chin is rounded transversely. The masseteric fossa is well 

 marked and defined anteriorly by a prominent ridge descending from the 

 front border of the coronoid process to the lower third of the side of the jaw. 



The ramus of the jaw would appear to have increased in depth and 

 assumed a more robust condition in the advance of age, for in those specimens 

 in -which the teeth are least abraded, the jaw is shallowest, and in that in which 

 they are most worn it is deepest. Specimens exhibiting the teeth in an 

 intermediate state of wear have the jaw of intermediate depth and strengtli 

 to the others. 



All the specimens represented in Figs 1, 3, 4, 6, and 7 I attribute to the 

 same species, notwithstanding the difference in the proportion of length lo 

 depth in the different ones. 



Two or three mental foramina occupy a slightly variable position beneath 

 the premolars. 



During the summer of 1871, Dr. Carter discovered, at Grizzly Buttes and 

 Lodge-Pole Trail, several specimens, consisting of fragments of upper jaws 

 with well-preserved teeth, which are of a size and form that would adapt 

 them to the lower-jaw specimens of Hyopsodus. 'I'he specimens from 

 Grizzly Buttes were accompanied by one of the lower-jaw specimens upon 

 wliich the latter was founded, and this looks sufficiently like several of them 

 in general appearance to have belonged to the same individual. 



One of the specimens represented in Fig. 18, Plate VI, contains a series 

 of three premolars and the succeeding molars. In advance of the series there 

 remains a portion of an alveolus which apparently belonged to another pre- 

 molar. If such is the case, the number of premolars would be the same as_ 

 in the lower jaw of Hyopsodus. 



The teeth (Figs. 18 to 22) increase in size from the first to the sixth, the 

 seventh being again reduced to the size of the fifth The second premolar 

 is inserted by a pair of fangs of which the posterior is wider than the other. 

 The succeeding premolars and molars are inserted with three fangs, of which 

 the inner one of the molars is a connate pair. 



Tiie crowns of the molars (Figs. I'J to 21) are quadrate, wider transversely, 



