DAKOTA AND NEBRASKA. 243 



of the sixth upper molar of the Tarija Mastodon, while that of the Tarabla Mastodon 

 measures scarcely fifteen centimetres fore and aft, and less than eight centimetres in 

 width at the fore part. 



The crown of the Tambla Mastodon tooth, figure 14, plate XXVII, presents four 

 divisions or ridges composed of transverse pairs of lobes. The summits of the latter 

 are all worn, and, with the exception of the outer lobe of the last pair, exhibit ex- 

 posed tracts of dentine. 



The outer lobes of the crown are smaller and of simpler form than the inner ones. 

 They are conical, wider transversely than fore and aft, convex and nearly vertical 

 externally, and sloping and depressed towards the middle in front and behind. 

 Their worn summits present transverse ellipses of dentine, of which the anterior one 

 had just joined that of the corresponding inner lobe. The third outer lobe is abnor- 

 mally undeveloped, is less in size than the one behind it, and appears simply as an 

 additional offset to the third inner lobe. The fourth outer lobe is a simple cone, worn 

 at the summit posteriorly in a sloping manner, but not so as to expose the dentine. 



The inner lobes of the crown at their internal semi-diameter are transversely 

 convex. They expand in the median line of the crown, contiguous to the outer lobes, 

 and form thick convex buttresses, which obstruct the middle of the transverse valleys. 

 In the anterior two of the inner lobes the summits are worn into broad and deeply 

 concave pits of dentine, bordered by thick enamel. The third inner lobe is connate 

 with the abnormally but partially developed outer lobe, and together at the worn 

 summit they exhibit a cruciform tract of exposed dentine. The fourth inner lobe 

 presents a minute cruciform tract of dentine on its summit. 



The contiguous buttresses of the second and third inner lobes, less worn than the 

 others, appear compounded in part with an intervening pair of mammillary accessory 

 lobules. From the less degree of development of the contiguous buttresses of the third 

 and fourth inner lobes, the posterior valley is less obstructed than the others. The 

 posterior buttress of the fourth inner lobe is larger than the anterior, and it, together 

 with an element of the basal ridge externally, forms a posterior convex prominence 

 or heel to the crown. The basal ridge was best developed at the fore part of the 

 crown, but thick elements of the same exist at the inner outlets of the anterior two 

 transverse valleys. 



The tooth is provided with three distinct fangs, of which the autero-external one 

 measures six and a half inches in length, following the curvature of its fore part. 



In a recent visit to Washington (May, 1869), in the Museum of the Smithsonian 

 Institution, I observed a molar tooth, from Nicaragua, Central America, presented 

 by Capt. Dow, which appears not to have pertained to the same species as the 

 former, b\it to the 2L amerkanus. The tooth I took to be a fifth of the upper jaw. 



