OCR Text |
Show 158 MR. ST. GEORGE MIVART ON MICRORHYNCHUS. [Mar. 13, inferior molars. It has four main cusps, the anterior of the two internal ones being opposite the interspace between the two external ones. A transverse ridge connects the postero-internal cusp with the postero-external one, and another connects the antero-external with the antero-internal one, in addition to which an oblique ridge connects the postero-external with the antero-internal cusp. The antero-external cusp is divided into two processes, the internal one of which is almost on a line with the two internal cusps, so that the tooth may be said to have two external and three internal cusps. The cingulum is very rudimentary within and little marked without. The anterior main cusps are subequal in height; but the postero-internal is higher than the postero-external cusp. The posterior part of the tooth is considerably broader than the anterior. The homologous tooth in Indris is quite similarly formed. Fig. 5. Left half of mandible. Scale, nat. size. The second lower molar is like the preceding, except that the transverse diameter of the anterior part equals that of the posterior part, that the antero-internal cusp is considerably more extended vertically, and that the supplementary anterior cusp is very much smaller or absent*. The same tooth of Indris is similar, except that the supplementary cusp and oblique ridge are both absent. The last lower molar, is like the second one; only the supplementary lobe is entirely obsolete, and there is a minute third posterior lobe at the middle of the posterior margin, making the tooth quinque-cuspid. There seems also to be no oblique ridge. In Indris the last lower molar is similar, except that the fifth cusp is more developed. inch. Length of the anterior upper incisor "08 Breadth ,, „ "03 Length of the posterior upper incisor '10 Breadth „ „ "05 Interspace between the two median incisors '17 Antero-posterior diameter of upper canine at its base '12 Extreme antero-posterior diameter of upper canine "15 Vertical extent of upper canine "15 Extreme width from within outwards '08 Antero-posterior diameter of anterior upper premolar at its base *16 * It is smaller on one side, and absent on the other side of the specimen described. |