210 ON THE EXTINCT MAMMALIA OF 



sion of the inner lobes, as seen in the first true molar, represented in figures 1, 5, 

 plate XXIV. In this condition the anterior deep pit of the antero-posterior valley 

 appears with its lining of enamel surrounded with dentine, and connected with the 

 enamel of the base of the antero-internal lobe by a narrow isthmus, as seen in the 

 first and second true molars represented in figures 1, 5, 6. 



An isolated upper last molar, represented in figure 7, plate XXIV, two-thirds the 

 size of nature, less worn than in the complete series above indicated, also exhibits a 

 less proportionate degree of development of the back part of its crown. 



Superior premolars. — The first upper premolar, figure 1, i^ plate XXIV, of Titanoihe- 

 rium, in the upper jaw specimen of Prof Hall's collection, is a small tooth about a 

 fourth the size of the succeeding one. The crown in its present condition is trilateral, 

 and is worn on the triturating surface so as to leave a broad exposed tract of dentine 

 sloping in a somewhat convex manner inward and forward to the base. The exter- 

 nal surface of enamel presents an indentation, indicating a pair of lobes as having 

 entered into the composition of the outer portion of the crown, as in the better 

 developed succeeding teeth. The triturating surface also at its back part presents a 

 triangular process of enamel dividing ofi" the inner portion of the crown, which in the 

 unworn condition formed a rudimental lobe. 



The succeeding three premolars, figure 1, 3, 4^5, of the same specimen, successively 

 increase in size, and have quadrate crowns with the transverse diameter exceeding 

 that from before backward, the disproportion successively increasing. In some speci- 

 mens the disproportion is greater than in others, as represented in figures 3, 4, in 

 comparison with the corresponding teeth in figure 1. 



The crowns of these premolars are constituted of four lobes as in the case of the 

 true molars, but they are less distinctly or separately developed. Indeed, the crowns 

 of the premolars appear like those of the true molars, reduced in proportions and 

 conjoined antero-posteriorly in such a manner that the inner and outer pairs of lobes 

 appear connate as a result. 



Externally the outer lobes are not separated by a prominent median buttress, as 

 in the true molars, but present a broad surface more resembling the condition of the 

 corresponding surface in the upper molars of Rhinoceros. The base and lateral 

 borders are somewhat swollen, and in some specimens a basal ridge exists. The 

 lobes terminate at the triturating border in angular points blunted by attrition. A 

 convexity or ridge, varying in the degree of prominence, descends to each point, and 

 an intervening depression defines the lobes. The internal faces of the outer lobes 

 form convex buttress-like prominences, separated from the inner lobes by an antero- 

 posterior zig-zag valley as in the true molars. 



The inner j)air of lobes of the crown are more or less completely connate. The 



