DAKOTA AND NEBRASKA. 247 



are proportionately longer or deeper and more cylindrico-conical or mammilliform 

 than in the plaster cast. They are also more deeply separated by the transverse 

 valleys, which are more contracted and angular, and at their outlet approach more 

 nearly the base of the crown. 



The anterior three divisions of the crown incline forward, and arc worn so as to 

 exhibit tracts of dentine at the summits of their component lobes. The fourth divi- 

 sion and the rudimental one sligbtly diverge from those in advance, or have a back- 

 ward inclination. In the cast, and also in the corresponding tooth of M. americanus, 

 all the divisions of the crown have a forward inclination. 



All the divisions of the crown are separated by a well-marked vertical fissure into 

 the usual pairs of lobes. Each lobe is further sub-divided by a distinct fissure into a 

 pair of lobules. The exterior lobules are the larger, prominently mammilliform, 

 and nearly alike, but the inner ones are more vertical than the outer ones. The 

 intervening lobules are contiguous, and appear compressed between the fore part of the 

 exterior lobules. 



In the anterior two divisions of the crown, the exposed dentinal tract of each lobe 

 or connate pair of lobules has an irregular dumb bell, or figure-of-eight outline. In 

 the third division, each lobule presents a minute dentinal pit at its worn summit. 

 The fourth division of the crown is composed like those in advance, and is propor- 

 tionately much better developed than the corresponding division in the plaster cast 

 and in the tooth of M. americanus. The heel or fifth lesser division of the crown is 

 composed of a pair of mammilliform lobes, about half the size of the exterior lobules 

 of the lobes in advance. 



The transverse valleys are narrower, deeper, and more angular than in the plaster 

 cast. The accessory eminences occupying and obstructing their middle are in trans- 

 verse pairs, and are mammilliform. The first pair are nearly equal, the inner being 

 the smaller, and their worn summits, with a central dentinal pit, reach the level of 

 the contiguous pair of divisions of the crown. Of the remaining pairs of accessory 

 eminences, the inner one in each is very small, while the larger ones successively 

 diminish in size. 



The anterior basal ridge is partly broken away, together with a portion of the in- 

 ternal lobule of the first division of the crown. The remaining portion of the former 

 is worn off very obliquely, and exhibits part of an interior tract of exposed dentine. 



The fore and aft diameter of the crown is five and a half inches; the greatest 

 width, which is at the bottom of the third ridge or division, is thirty-one lines. The 

 height of the unworn fourth division of the crown externally is thirty-two lines. 



Since writing the above, I have received for examination, from the Smithsonian 

 Institution, two fragments of Mastodon teeth found at Tarboro, North Carolina. One 

 is the posterior half of a last inferior molar, and is represented in figure 16, plate 



