| OCR Text |
Show 1866.] MR. ST. GEORGE MIVART ON MICRORHYNCHUS. 157 mon in the Lemuroidea. Externally their crowns are convex from above downwards, but concave from side to side at their upper halves, which are longitudinally grooved. Internally their crowns are slightly concave from above downwards, and strongly convex transversely, there being, in fact, a median longitudinal prominence extending from the base to the apex of each tooth. The lateral margins of each incisor (on each side of the median prominence) are probably prolongations upwards of the rudimentary basal cingulum*. The lower incisors of Indris and Propithecus are quite similarly formed; but the marked external longitudinal grooves are quite peculiar to the Indrisinar amongst Lemuroids. The lower canines are like the incisors, except that they are rather broader, that the external grooves are less marked, and that the lateral prolongations upwards of the basal talon are more so. In Indris the same conformation exists, save only that the inner surface has a more marked and wider groove, owing to the greater development of the lateral prolongations of the talon. The anterior lower premolar shows a tendency to assume the elongated and anteriorly produced form of the incisors and canines, yet it is the most vertically extended tooth in the lower jaw. In shape it is like the anterior upper premolar, only more extended forwards. It is unicuspidate, the cusp being the extremity of a strongly marked median longitudinal prominence. There is a slight basilar posterior process, and a strongly marked anterior one developed from the middle of the anterior margin. Both these appear to be productions of the cingulum, which exists within, but is scarcely traceable externally. This tooth differs from the corresponding one of all other Lemuroids except Indris and Propithecus. k Fig. 4. Inside of left lower premolars. Scale, twice nat. size. The posterior lower premolar is like the tooth just described, except that the anterior process is relatively more marked, the apex less produced, and that the median longitudinal ridge is so prominent as to project upwards above the external margin of the tooth, the grinding-surface presenting a marked antero-posteriorly directed groove at its hinder half. This tooth closely resembles its homologue in Indris; it has a certain resemblance to the second lower premolars of Galago, especially of G. sennaarensis. The first lower molar is the longest of the inferior grinders, from before backwards ; and there is more difference in size between it and the second premolar than there is between any other two contiguous * As described by Prof. Huxley in Arctocebus (see P. Z. S. 1864, p. 322). |
