92 . KHINOCEROS. 



abruptly descends half the length of the crown (6), then, proceeding inward, it 

 envelops the base of the postero-internal lobe, and internall}- it ascends to the base 

 of the autero-internal lobe, and winds anteriorly to the antero-external margin of 

 the crown, and then makes an abrupt ascent to the base externally (7, 8). 



The anterior and posterior valleys in all the premolars ai'e deep culs-de-sac with 

 shelving sides (5). 



When the molar teeth have had one-half their crown worn away in mastication 

 they are hardly recognizable in those which have not been subjected to trituration. 

 Comparatively with one another, they of course suffer attrition most in the order of 

 their succession, and this, judging from the specimen in Dr. Owen's collection, in 

 which the seventh molar is only partially protruded, may be determined to occur 

 in the following manner. After the temporary teeth, the fifth molar is protruded, 

 and in the permanent series appears most worn; then succeed the first to the fourth 

 permanent molars, then the sixth, and finally the seventh (13). 



In the specimen of the skull containing all the molars, presented by Capt. S. Van 

 Vliet to the Smithsonian Institution, these teeth are worn about one-half away, and 

 exhibit very strikingly the transformation of form produced by attrition. (XV. 3.) 



The enamelled grinding surface of the fifth molar, except a short inlet consti- 

 tuting the entrance of what was the principal valley, has been completely oblite- 

 rated. The exposed dentinal surface is concave, and bordered by enamel, except 

 anteriorly and posteriorly, where it also appears to have been removed, probably 

 from the combined influence of long-continued pressure and friction from the 

 contiguous teeth. 



In the sixth molar, the exposed dentinal surface is more deeply bilobed internally 

 than in the former; or, in other words, a longer tract of enamel remains from the 

 anterior valley ; and farther, almost the whole of the bottom of the posterior valley 

 yet remains. 



The seventh molar, from its being the last to take its position in the functional 

 series, is worn less than any of the others. Its valley remains entire, except that 

 it is rendered a little more shallow, from the summits of the lobes which embrace 

 it being worn off. The exposed dentinal surface presents an irregular V-shaped 

 figure, with the apex and extremities of its arms bifurcated. 



The second to the fourth molars inclusive jaresent nearly square dentinal surfaces 

 bordered with enamel, bilobed internall3', and possessing, each, two trilateral pits 

 of enamel, the remains of the valle3^s. The central pit is the larger, and has 

 convex sides and rounded angles; and the smaller pit is in contact with the poste- 

 rior border of the teeth. 



The exposed dentinal surface of the first premolar, in the specimen, upon one 

 side of the jaw, has two small circular pits of enamel, and on the other, a single 

 trilateral pit, which remains from the posterior valley; and in both teeth a cul-de- 

 sac in connection with the internal border exists before and behind the rudimentary 

 antero-internal lobe. 



When the enamelled triturating surfaces of the molars are completely obliterated 

 by mastication (1, 2), as is the case in the specimen of a skull of a very old animal 

 in the collection of Dr. Owen, the exposed dentinal surfaces are quadrate and bilobed 



