170 DK. G. ELLIOT SMITH ON THE BEAIN 



oi Indris hrevicaudatus \yio. 253 in the Museum of the Eoyal College of Surgeons, 

 London], which I have figured in my previous memoir (i, fig. 30, p. 353). 



The deep backwardly-prolonged Sylvian fissures, the parallel and the extensive deep 

 postlateral sulci call for no special account, for they merely reproduce the arrangement 

 found in Indris and often also in Propithecus. But the intraparietal and inferior 

 frontal sulci call for more detailed study, if for no other reason, because they are 

 subject to so much variability in the extant Indrisinae. 



Text-fig. 59. 



f. e. s. uitrapdy, 



\ j \jiss. Syl. 



&franfi. inf. y\', \ i ^ 



v-s lunat-. 



bulb, olf.- 



s.orb. T\ y^ ^^ 



afhont. orbit. ~'~^ 



Diagram of the left lateral aspect of the brain of Meso2»-o^itheeus pithecoides. Natural size. 



There is one feature that is curiously distinctive of the Indrisinae ; it is the imperfect 

 development of the intraparietal [lateral] sulcus. In the genus Lemur and, in fact, 

 almost all Primates this sulcus is deep and long ; and in most other mammals this is 

 one of the most constant and best developed furrows in the neopalUum, In the 

 Indrisinge, however, it is often represented merely by one or two shallow depressions 

 or at most a short sulcus, quite unlike the long deep furrow usually found in the genus 

 Lemur. The backward extension and deepening of the Sylvian fissure, which are so 

 marked in the Indrisinae and especially in Propitliecus, are present also in this extinct 

 genus. An analogous but not altogether identical overgrowth of the Sylvian at the 

 expense of the intraparietal sulcus is found also among the Prosimise in Nycticebus and 

 in several of the Cebidse — Chrysothrix, Nyctqnthecus, Aotus, and Alouatta. In 

 Mesopropithecus the intraparietal sulcus was a very short shallow depression, not more 

 than 4 mm. long, placed just behind the mid-point of the hemisphere about 7 mm. 

 from the mesial plane. 



A few millimetres further forward there was a short slightly oblique sulcus "/." 

 The inferior frontal [coronal] sulcus was long and deep with an upturned caudal 

 extremity (fig. 59, e) as in many Lemurs and the specimen of Indris described by me 

 /• I, p. 353). In the brain of the young specimen of Projnthecus diadema described by 

 me (2) there is a similar inferior frontal sulcus, but in my specimen of the brain of 

 Propithecus coquereli (fig. 57), as also in Grandidier's and Milne-Ed wards's specimen of 



