OK THE MOLAR SERIES OE RHINOCEROS TICHORIIIHUS, 527 
posterior border of tbe tooth, and holds the same relation to the pos¬ 
terior that the Median Collis does to the Anterior Valley. In the 
upper molars this typical arrangement is concealed by the great de¬ 
velopment of the Anterior Valley and the Median Collis, at the 
expense of the Posterior Valley and Collis, which are thrust, as it 
were, backwards, and excluded from a share in the inner surface of 
the tooth. The entrance of the Posterior Valley is also so close to 
the summit of the crown, that it is soon worn away, leaving the valley 
as an island of enamel, surrounded by dentine. A series of teeth in 
my possession shows the stages by which the typical form is obscured. 
In it the posterior island of enamel of the Premolars gradually 
increases in size from before backwards, through the large partially- 
open valley of m 2 into the indisputable posterior valley of m 3. 
"Without this evidence, I should have hesitated in adopting Brandt’s 
views of the homologies on the evidence he gives. 
The external surface bears costas (K), or ribs, more or less vertical, 
divided from each other by sulci, or grooves. The deepest and most 
persistent of these is the Median or Master Groove (I), dividing the 
surface vertically into two areas ( M. N. ), the anterior and the 
posterior. The former of these always bears two costae, which, in the 
lower molars, are sometimes nearly obsolete. 
The enamel bears rough vertical rngss, more or less developed, and 
never parallel. Besides these is a set of fine vertical and parallel strife, 
which in the older teeth are almost obliterated by the more strongly- 
marked series. Also, and especially in the Premolars, is a third set of 
markings, linear, horizontal, and very superficial, only to be seen in the 
young teeth, and forming, with the two vertical sets, a faint reticulated 
pattern. In the milk molars, the first of these three series of mark¬ 
ings is very fine, the second barely visible, and the third absent from 
all that I have examined. 
§ 4. Dehtal Formula. —With reference to the number of the 
Permanent molar series dentition, the great authorities are by no 
means agreed. On the one hand, Cuvier, on the faith of a letter 
from Adrien Camper,* and after him Blainville,f maintain that R. 
tichorhinm has a full complement of Premolars ; while, on the other, 
Pallas J and Fischer § doubt it: and Brandt, || after carefully weighing 
the evidence, concludes that the first Premolar is always absent 
from the adult. It must be remarked that neither Cuvier nor Blain- 
ville ever saw the first premolar, the existence of which they assume. 
Professor Owen, indeed, figures and describes^ a tooth as being that 
in question, mistaking the deciduous for the permanent dentition in 
the two jaws from Thame and Lawford in the Oxford Museum. But, 
* Ossemens Fossiles, Yol. ii. Pt. I. p. 61,1822. f Osteograph. Rhinoceros, p, lo7. 
$ Novi. Comment. Petropol. Tom. xvii. 
§ Fischer, Oryctographie de Moscou; p. 114. 
|| Brandt, op. cit. p. 325. 
4 Brit. Foss. Mam. (1846), pp. 337—342, 363-4, Figs. 128, 137. 
N. II. R.—1863. 2 N 
