DAKOTA AND NEBRASKA. 307 



expand and are conjoined at the bottom of the crown, forming together a pair of 

 arches bounding the external surfaces of the outer lobes. These surfaces are nearly- 

 flat, and are divided by a conspicuous median ridge. The internal sui'faces of the 

 outer lobes are prominently or almost angularly convex. 



The inner lobes of the crown are simply conical, wider transversely than fore and 

 aft, and with the anterior slightly larger than the posterior. 



The median lobes are not more than half the size of the principal ones, and appear 

 as prominent folds curving from the inner lobes outwardly to the anterior face of the 

 outer lobes. They are bulging near their commencement, so that the summit in the 

 worn condition presents a curved clavate form. 



Constituent elements of a basal ridge exist at the fore and back parts of the crown, 

 and at the outlet of the valley separating the inner lobes. 



In the interval posteriorly between the back inner and outer lobes there exists a 

 tubercle, which, in association with the contiguous portion of the basal ridge, assumes 

 the dignity of a sub-lobe. 



In the first large molar of the upper series, the anterior buttress is more distinct or 

 separate than in the others, though it is not so large. 



In comparing the upper molars of A. Bairdi with those of a series of A. aurelian- 

 ense, the differences observable are of the slightest character. The inner lobes are 

 slightly wider in relation to their fore and aft diameter ; the external median ridge 

 of the outer lobes is proportionately better developed ; and the element of the basal 

 ridge at the outlet of the valley between the inner lobes is larger. 



In the process of mastication, from attrition, tracts of dentine become exposed at 

 the summits of all the lobes. These at first form a pair of crescents on the outer 

 lobes, oval islets on the inner lobes, and clavate ones on the median lobes. As 

 abrasion continues, the dentinal tracts widen and become continuous. Wlien the 

 crowns are about half worn away they exhibit broad surfaces of dentine, with small, 

 median, crescentic, shallow enamel lakes, with long curving, narrow, enamel outlets 

 between the remains of the inner lobes internally. Subsequently the narrow enamel 

 outlets are obliterated and then follow the median enamel lakes, leaving broad ex- 

 panses of dentine bordered by enamel. 



Of the lower molars, the first is a small tooth inserted by a single fang. It is lost 

 in all the specimens under examination, though its alveolus is retained in several. 



The six large lower molars have oblong quadrate crowns, with the breadth about 

 twice the transverse width, and about a third greater than the length. They are 

 composed of an outer fore and aft pair of principal lobes, with an inner pair of second- 

 ary lobes, connate with the former. The last of the series has an additional less well 

 developed principal lobe. Each tooth has two fangs, of which the back one in the 

 last molar is a connate pair The principal lobes of the crown are slightly oblique in 



