Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/586

 & 2, i) is defined by a cleft on the outer side of the tooth, but not on the inner side, fig. 3 ; here the abraded surfaces of ridges 1 and 2 are blended by wear into a common hollow field of smooth dentine (fig. l, a). There is a slight constriction near the part where the worn surface of the first ridge blends with that of the second ; and this constriction, which may be detected in the succeeding ridges, I take to be a trace of that stronger one which more completely divides the transverse coronal ridge in the molars of better Mastodons into an inner and an outer part. A well marked tubercle (figs. 1 and 2, f) projects at the outer side of the base of the first ridge, l, near the interspace between that and the second ridge. The second field of abrasion (ib. ib. 2) although it broadens inwards to the common hollow, shows, before losing its individuality, a similar indication of constriction, or reciprocal inbending of the enamel boundary. The same indication, though feeble, is obvious in the succeeding- ridges (3, 4, 5, a), which, by the unequal working of the lower grinder, show a broader field of dentine as they pass inward. The third and fourth ridges, which are entire, show their slightly undulated course from the outer to the inner side, which is lower and more worn. The ridge at first inclines a little backward, then, at the indication of constriction, bends forward, and finally resumes the transverse course to the inner, lower and more worn side of the tooth.

Now this character is not shown in the Mastodon elephantoides, Clift, of which the antepenultimate upper molar (m 1) is figured, of the natural size, in plate 39. fig. 6, loc. cit., — nor in the homologous tooth of the same species, also from Ava, figured (one-third natural size) as of Elephas Cliftii, in the ' Fauna Antiqua Sivalensis' of Falconer and Cautley, pl. 30. fig. 2.

From the general conformity of character of the transverse ridges in the last three molars (m 1, m 2, m 3) of this species, it is unlikely that so marked a difference of course and configuration of the ridges should exist in the second grinder, answering to d 3, of the same species.

Nevertheless in the number of ridges in a given tract of the grinding-surface, in their height and breadth of base, and in the absence of intervening cement, the conformity of the Chinese molar with the grinders of the Mastodon elephantoides is close. The enamel also shows the same vertical linear impressions and ridges, by which we may reckon that the summit (say, of the fourth ridge in the tooth here described), if it were unworn, might be cleft into from thirteen to fifteen small mamillae.

This structure is well shown in the full-sized figure of the upper molar (m 1) of Mastodon elephantoides, Clift (in plate 39. fig. 6, Trans. Geol. Soc. vol. ii. second series).

In a tooth of an allied species from the Siwalik tertiaries, homologous with the specimen under description, figured in the ' Fauna Antiqua Sivalensis,' pl. 29. figs. 3, 3 a, as of Elephas bombifrons, but subsequently referred by Falconer to his Elephas (Stegodon) insignis*,


 * Palaeontological Memoirs, vol. i. p. 459, in description of plate 29.