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Show 1905.] THERIODOXT REPTILE DIADEMODOX. 97 tooth on the right side shows the anterior serrations, and the second tooth on the right side bears indications of the posterior serrations. At the base of the posterior edge of the outer cusp there is seen in tlu'ee of the teeth what looks like a very small secondary cusp, but this may be merely a ridge which joins the outer and the inner large cusps. It will thus be seen that the premolars have crowns very like those of the human bicusps, but differing in having the cusps more marked and with small serrations at least on the outer side. Each tooth that has been displayed has a single cylindrical root. All seven molars on the right side, and all except the second on the left, are preserved. The first four are round teeth with flat tops. It is probable that the flattening is largely due to wear, and the enamel seems to have been worn oft' the tops of at least the first four. In the fifth much of the enamel is worn off, but in the sixth and seventh teeth there is no evidence of wear. In removing the matrix, which is a fine-grained calcareous sandstone, the processes of the enamel were found very apt to adhere to the matrix and to become detached from the dentine, but fortunately almost every feature of the sixth and seventh molars is preserved on either one side or the other. The sixth molar, when viewed from above, has an almost circular crown, being only very slightly broader transversely than antero-posteriorly. There is a single prominent cusp on the middle of the outer edge, a.nd a second similar but slightly smaller cusp on the middle of the inner edge. Between these two there is a well-developed, slightly concave ridge interrupted in the middle by a slight elevation, and dividing the crown of the tooth into almost equal anterior and posterior portions. The anterior portion, which is moderately flat, slopes down from the median ridge to the anterior edge, which is slightly elevated and supported by three small cusps. These cusps are arranged as follows :- one between the middle of the anterior edge and the large external cusp, but nearer to the middle line; the other two immediately in front of the large internal cusp. The posterior half of the crown is somewhat similar to the anterior, but rather more concave; it has the posterior edge supported by a series of small cusps. On the two sides the arrangement is slightly different : on the right side there are four subequal cusps close together; on the left side one larger cusp takes the place of the centre two. The seventh molar is considerably smaller than the sixth, but fairly similar in structure. The posterior part of the crown is narrower than the front part. Thei'e are two cusps on the anterior edge, and two, with possibly a small third, on the posterior edge. When the lower molars are compared with the upper molars described by Seeley, it is seen that they fit satisfactorily-the transverse ridge on the lower molars fitting between two upper molars, and the ridge of the upper between two lower molars. Proc. Zooe. Soc.- 1905, Y ol. I. Xo. YTI. 7 |