DAKOTA AND NEBRASKA. 209 



median in position. It is in the fonn of a broad cone projecting from a concave 

 basin, except where it contr'ibutes to the swollen base of the crown internally. See 

 figures 1, 5, 6, 7 



The posterior of the inner lobes resembles that in advance, as seen in the first and 

 second molars represented in figures 5 and 6, or in other specimens it assumes more 

 or less the form of a three-sided pjTamid, as represented in figures 1, 7. In the 

 latter condition the three sides are bounded by as many ridges, of which two con- 

 tribute to the posterior and internal borders of the crown. The third ridge proceeds 

 obliquely from the point of the pj^ramidal lobe outward and forward to terminate 

 opposite the middle of the base of the postero-estemal lobe of the crown. The inner 

 face of the pyramid is vertical and convex; the other two faces, directed forward and 

 outward, are convex in the fii'st true molar, sloping planes in the second, and concave 

 in the last molar. 



In advance of the antero-intemal lobe a thick ridge curves from its l^ase to the 

 fore part of the antero-external lobe. This ridge, a constituent element of a basal 

 ridge, successively enlarges in passing from the first to the last of the true molars, 

 occupying the angular interval between the anterior two lobes of the crown. In the 

 second and third molars it almost assumes the importance of an additional lobe. 

 Though holding nearly the position of the anterior of the three internal lobes in the 

 true molars of Anoplotherium, it appears not to be homologous with it, but with the 

 element of a basal ridge at the bottom of that lobe. It corresponds with a similar 

 thick ridge in the same relative position in the true molars of Chalixiotherium. 



The antero-posterior valley of the crown of the upper true molars of Titanotherium 

 follows the zig-zag course of the base of the inner sides of the outer lobes, and com- 

 municates with that surrounding the base of the antero-intemal lobe. Its bottom is 

 rather irregular, being more or less interrupted by pits and tubercular processes. A 

 deep pit occupies the valley at the bottoru of the postero-internal face of the antero- 

 external lobe, and a shallower and less distinct one is situated m a corresponding 

 position ia relation with the postero-external lobe. 



In the trituration to which the upper true molars of TitanotJiermm were subjected 

 in mastication, the acute crescentic summits of the outer lobes were first worn away. 

 As the enamel was removed, W-shaped tracts of dentine were exposed, bordered by 

 the former substance. The dentinal tracts gi-aduallj^ widened and extended to the 

 bottom of the antero-posterior valley of the crown, and the inner enameled sides of 

 the outer lobes were thus obhterated. (See figures Ij.e.v, 5, 6, 7, plate XXIV.) 

 It was not until about this stage of wearuig that the dentuie began to be exposed 

 at the apices of the inner pair of lobes, and it was only after the enamel had been 

 worn away from the outer lobes and across the antero-posterior valley of the crown 

 that considerable circular or ovoidal islets of dentine became apparent by the abra- 



27 



