RHINOCEROS. 93 
internally, a little broader transversely than antero-posteriorly, smooth and more 
or less depressed. Most of the crowns are bordered with enamel only at their 
internal portion; it having entirely disappeared upon the true molars externally, 
and probably also upon the premolars, but these, in the specimen, are too much 
broken at their outer part to judge. Between the teeth also the enamel has par- 
tially and in many positions entirely disappeared, so that the dentinal masticating 
surfaces are separated only by the interstices of the crowns. 
Inferior Molars.—(P1. XIV. 2, 3.) The normal number of lower molar teeth of 
Rhinoceros is seven, as in the upper jaw; but in the only specimen of the lower jaw 
of Rhinoceros Nebrascensis which we possess, the number of molars on each side is 
six; the first having been long shed and its alveolus entirely obliterated. 
In form, the inferior molars resemble closely those of all other species of Rhinoceros. 
All have a basal ridge surrounding them, except where it has been obliterated, in 
the course of time, by pressure and friction between the teeth. 
The second molar of the normal series, in outline transversely, presents an isos- 
celes triangle, but, like the others, it is constituted of two distinct lobes, of which 
the anterior is so much compressed laterally as to lose the crescentoid form. 
In the specimen of the lower jaw accompanying the skull obtained by Captain 
Van Vliet, the crowns of the molars are considerably worn away. The fifth molar 
has its two crescents nearly obliterated; the sixth is less worn; and then follows in 
succession the fourth to the second, and then the seventh. In the latter, the exposed 
dentinal surfaces of the crescentic lobes are distinct, but in all the others they have 
become confluent. From the long continuance of pressure and friction the enamel 
has disappeared where the teeth are in contact, except between the anterior and 
the posterior two. 
In a small fragment of lower jaw accompanying the skull of a very old animal 
in Dr. Owen’s collection, containing the fifth molar, the enamel of this has been 
worn away, except a very small portion at each posterior angle, and the masti- 
cating surface of dentine in outline has the form of the figure 8, and is trans- 
versely convex and antero-posteriorly concave. Small fragments of the teeth in 
advance and behind, in the same specimen, indicate them not to have been as much 
worn, so that the nearly entire tooth is probably the first of the true molar series. 
Temporary Molars.—The posterior three temporary molars, as I suppose them 
to be, contained in a fragment of the upper jaw, are of about the same size 
as their permanent successors (XIV. 14). They are more square, or are less 
contracted and convex internally, and the inner lobes are more equal in size, and 
do not become confluent internally. In consequence of the latter arrangement, the 
principal valleys are open at their entrance to the bases of the lobes, and in the 
third and fourth molars they deepen from without inward, as in the case of the 
two anterior permanent true molars. In these two temporary teeth, also, the poste- 
rior valleys are deeper at their outer end than those anterior.. In the second molar, 
the bottoms of both valleys are nearly uniform in their depth throughout and with 
each other. The basal ridge is horizontal upon the inner side of the temporary 
molars, and in front and behind very gradually descends to the external margins 
of the crown. 
13 
