99 RHINOCEROS. 
a 
abruptly descends half the length of the crown (6), then, proceeding inward, it 
envelops the base of the postero-internal lobe, and internally it ascends to the base 
of the antero-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 are 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 present nearly square dentinal surfaces 
bordered with enamel, bilobed internally, and possessing, each, two trilateral pits 
of enamel, the remains of the valleys. 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 
