Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/644

462 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 27, that at the bottom a portion of the sinus is isolated. The uncus arises from the posterior column at a very acute angle ; but owing to its rapidly making a curve outwards, its general direction is nearly parallel with that of the column ; and the short reentering angle is occupied by a slender columella, d, rising from the bottom of the sinus. A short crista (" anterior combing -plate "), e, projects into the sinus on its outer side, and in a line nearly vertical to the uncus, with which, however, it is in no way connected. What remains of the dorsum or " outer lamina " shows no elevation of the first or angular costa (f), but a considerable elevation of the second (g) ; the remainder of the surface is evenly undulated, without any distinct indication of a fourth costa. The anterior column is widely expanded at its inner end, with a rather deep sinuosity on its anterior aspect (h). The enamel throughout is very thin, and where the surface is exposed it is smooth. Towards the base of the dorsum there is a thin irregular coating of cementum.

In the opposite tooth, no. 877 (Fig. 2), we are fortunately furnished with exactly the parts that are wanting in no. 878, viz. the posterior column, &c. The tooth is so much worn that the posterior sinus (" valley ") is represented merely by a small circular pit (i): it is consequently impossible to ascertain whether the hinder vallum (" third collis ") was cuspidate or emarginate. In all other respects the tooth presents exactly the same characters as its opposite fellow, except that the entrance of the median sinus is furnished with only a single cusp, and that, owing to its being rather more worn, the uncus extends quite across the sinus, and becomes confluent with the anterior column, so as completely to isolate the external portion of the sinus, and produce a third pit or fossette. But it will be observed that this fossette is not formed in the same way as the peculiar "tichorhine pit" (that is, by the coalition, ab initio, of the uncus and crista), but simply, as not unfrequently occurs, by the prolongation at the base of the former, so that it reaches as far as the anterior column — a circumstance obviously of little moment as a character, since, as we here see, it may be said to exist on one side and not on the other. It may also be added that a similar prolongation of the uncus has been previously noticed in teeth of R. leptorhinus, an instance of which is afforded in pl. 51. fig. 4 of the ' Ossemens Fossiles,' which plainly represents, as it seems to me, a tooth of R. leptorhinus, and not of R. hemitoechus as supposed by Prof. Owen. And a similar instance is shown in the tooth figured by Mr. W. B. Dawkins (l. c. p. 410, fig. 10), I believe, from Grays Thurrock. And it is important to remark the occasional possible occurrence of this peculiarity, since M. Christol, in his definition of R. megarhinus, expressly says, " le crochet de leur colline posterieure ne se joint jamais a l'anterieure " *.

similar occurrence in a molar of R. hemitoechus, and remarks: — "That this peculiar confluence of the crochet with the anterior barrel is abnormal in the true molars, is proved by the extreme rarity of the instances which have been observed of it in any species of Rhinoceros." He then cites Cuvier's figure, to
 * Dr. Falconer also (op. cit. ii. p. 336, pl. xviii. fig. 5) notices and figures a