1866. J MR. ST. GEORGE MIVART ON MICRORHYNCHUS. 157 



mon iii 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 cin- 

 gulum*. 



The lower incisors of Indris and Projnthecus are quite similarly 

 formed ; but the marked external longitudinal grooves are quite pe- 

 culiar to the Indrisince 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 for- 

 wards. It is unicuspidate, the cusp being the extremity of a strongly 

 marked mediau 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 pro- 

 ductions of the cingulum, which exists within, but is scarcely trace- 

 able externally. This tooth differs from the corresponding one of 

 all other Lemuroids except Indris and Propithecus. 



Fig. 4. 



Inside of left lower premolars. Scale, twice nat. size. 



The posterior lower premolar is like the tooth just described, ex- 

 cept 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 cer- 

 tain resemblance to the second lower premolars of Gulago, 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). 



