208 ON THE EXTINCT MAMMALIA OF 



jecting outwardly and a little in advance of the back part of the contiguous tooth. 

 The upper premolars appear as contracted or less well-developed forms after the 

 pattern of the true molars. Their outer sides are placed together nearly on the same 



line. 



The upper canines are sejDarated from the molars by a comparatively short hiatus. 



The upper incisors follow close upon the canines, and those of the two sides are 

 separated by a concave notch, about half an inch in width, in the fossil under 

 examination. 



The lower molar teeth form a continuous row, and are also separated from the 

 canines by a small hiatus. 



Thus it appears that the teeth of TitanotJierium form rows in each jaw almost as 

 close as in Anoj^Iotherium, small intervals existing between the canines and molars, 

 and between the incisors of the two sides, in consequence of the absence of the first of 

 the usual number of these latter teeth as existing in Anoplothermm and many other 

 animals. 



/Siqjerior true molais. — The upper true molars of TUanotherium, figures 1, 5, 6, 7, 

 plate XXIV, have broad square crowns, which successively increase in size from first 

 to last. In the first the transverse diameter exceeds the antero-posterior, in the 

 second the diameters are nearly equal, and in the third the antero-posterior exceeds 

 the transverse. 



The crowns are composed of four principal lobes, as in Chalicotherium. The outer 

 pair of lobes resemble in their general form and relationship the corresponding ones 

 of the last-named genus, those of Anoj^hfherium, of the recent Deer, &c. Externally 

 they present a pair of inclined planes sloping downward and inward, and ending at 

 the triturating border in angular points. Laterally these planes are bounded by 

 thick, convex, buttress-like ridges, which expand in an arching manner at the base 

 of the lobes. The posterior buttress is less well-developed than the others, especially 

 in the first and second true molars. (See figures 1, 2, 5, 6, 7.) The anterior of 

 the two planes just mentioned presents a comparatively feeble median longitudinal 

 ridge, which is absent in the posterior plane. The base of the outer lobes externally 

 is somewhat thickened, and in most of the true molars under examination is devoid 

 of a distinct basal ridge. In some specimens, however, a conspicuous basal ridge 

 exists, as in the first and second molars represented in figures 5 and 6, and as 

 well seen in the fragments represented in figures 11 — 13, plate xvii, of the 

 Ancient Fauna of Nebraska. 



In the unworn teeth the inner surfaces of the outer lobes are transversely convex, 

 and slope from their bottom with a feeble concavity towards the triturating border. 



Of the inner lobes of the crown the anterior is rather the larger, and is nearly 



