DAKOTA AND NEBRASKA. 249 



in M. americanus. The summits of the lobes in each division of the crown approach 

 more nearly or incline more towards each other, the bottoms of the valleys are more 

 concave, and their middle is obstructed by comparatively small accessory eminences, 

 associated with the buttress-like ridges of the more inclined lobes. The posterior 

 basal ridge is produced into a single median conical eminence. The summits of the 

 anterior lobes of the specimen are worn at their fore part, but not sufficiently to 

 expose the dentine. The posterior lobes are unworn. 



The width of the specimen at the base of the posterior division of the crown is 

 forty lines, and at the base of the division in advance thirty-eight lines. 



Mastodon mirificus. 



Among the most interesting discoveries of Dr. Hayden in Nebraska are the remains 

 of a species of Mastodon, unquestionably distinct from those which had been so long 

 familiar to naturalists as pertaining to the M. americanus. 



The most characteristic specimen of the newly discovered species of Mastodon, dis- 

 tinguished by the above name, consists of the greater part of both rami of the lower 

 jaw, each containing the last molar tooth. It was found on the Loup Fork of the 

 Platte River, though remains, apparently of the same species, were observed by Dr. 

 Hayden on the Niobrara River. The left ramus of the specimen is represented in 

 figures 1, 2, plate XXV. 



The lower jaw of M. mirificus is smaller than that of M. ohioticus, being both 

 shorter and of less depth, but is proportionately of rather greater breadth posteriorly. 

 The form is nearly the same, but the rami are more convex externally, and the 

 alveolar border slopes backward. 



The specimen exhibits no trace of having been provided with incisor tusks. 



A single tooth on each side, the last of the molar series, occupies the alveolar 

 border of the jaw, and it has so completely replaced those which preceded it in 

 functional position that hardly a trace of their sockets is left. In advance of the 

 molar the border forms a prominent carina-like ridge, pursuing a sigmoid course to 

 the elongated chin. 



The crown of the sixth molar of M. mirificus exhibits six divisions or ridges, sepa- 

 rated by comparatively narrow valleys, and composed, in the usual manner, of trans- 

 verse pairs of lobes. 



A slight disproportion exists between the two teeth in the specimen ; that of the 

 right side measures eight and a half inches fore and aft and forty-one lines wide at 

 the base of the third division ; that of the left side measures eight and three-quarter 

 inches fore and aft, and has measured about thirty-eight lines wide at the base of the 



third division. 



32 



