DAKOTA AND NEBRASKA. 211 



anterior lobe is much larger than the posterior. Together they form an oblong 

 conoidal eminence, gradually narrowing from before backward, but rather abruptly 

 enlarging at the posterior extremity. The inner lobes are enclosed by a stout basal 

 ridge, as represented in figures 3, 4, plate XXIV, and also in figures 1 — 7, plate 

 xvii, of the Ancient Fauna of Nebraska, or the ridge may be interrupted internally 

 as seen in figure 1, 2, 3, 4, plate XXIV. 



The antero-posterior valley of the crowns of the three premolars under considera- 

 tion resembles that of the true molars, and, like it, is interrupted by two pits, 

 occujDying the same relative position. The posterior pit is much deeper than the 

 corresponding one in the true molars, and approaches in this respect the anterior pit. 



In the trituration to which the pi'emolars were subjected in mastication they 

 passed through the same steps as the true molars. As the outer lobes wore away, 

 gradually widening W-like tracts of dentine encroached upon the antero-posterior 

 valley. As the inner connate lobes were worn away, a clavate tract of dentine be- 

 came exposed, which, gradually widening from abrasion, became continuous by its 

 anterior broader end with the dentinal tract of the antero-external lobe across the 

 antero-postei'ior valley. 



The upper true molars of Titanoiherium bear a near resemblance to those of Chali- 

 coiherium, but the premolars are much better developed in their relation with the 

 former than in this genus. 



Inferior true molars. — Of lower true molars of Titanotherium the specimens already 

 described in the Ancient Fauna of Nebraska are the best preserved I have had the 

 opportunity of examining. (See figures 1, 2, 3, 8, 9, 10, plate xvi, and figures 8, 9, 

 10, plate xvii, of the Ancient Fauna of Nebraska.) 



The lower true molars of Titanotherium are constructed upon the same pattern as 

 those of Palceotherium, Anoplotherium, CJialicotlwrium and AnchitJierium. The 

 crowns of the first and second are each composed of a pair of lobes ; that of the last 

 possesses an additional less well-developed lobe. 



The inner side of the crown of the true molars forms a vertical, somewhat uneven 

 plane, divided towards the triturating border into angular points separated by 

 angular notches." In the first and second molars there are three of these points, 

 with two intervening notches ; in the last one, four points and three notches. 



The constituent lobes of the crown are demi-conoidal and confluent internally and 

 at the base. Externally they are sloping and transversely convex, and are separated 

 by deep angular valleys. In this position they are bounded by a basal ridge, usually 

 continuous, but sometunes nearly interrupted where the lobes are most prominent. 

 In the unworn teeth the summits of the lobes ai'e acute and angularly crescentoid, 

 and they ascend to a median point externally and terminate in the angular points 



