86 PROF. G. ELLIOT SMITH ON 



small, and is far removed both from the coronal (sco.) and post- 

 lateral ispl.) sulci. To compensate for its diminutive size, the 

 suprasylvian sulcus (ss.) is prolonged far back into a hook-like 

 extremity ; and on the right hemisphere it almost fuses vpith the 

 lateral extremity of the post-lateral sulcus (spl.). It is instructive 

 to recall in this connection that in those Apes, such as the 

 Lemurine Dourocoli (AoUcs felinus), in which the sulcus iutra- 

 parietalis becomes confluent with the suprasylvian sulcus, a 

 small furrow persists in the position where the anterior end of 

 the intraparietal (lateral) sulcus would be found if the fusion 

 had not taken place {vide fig. 229, p. 391, Catalogue of the Koyal 

 College of Surgeons, 2nd edition, vol. ii.). This seems to point 

 to the conclusion that in those Lemurs (e. g. Nycticebus) and 

 Apes (many of the Cebidse) in which the suprasylvian (Sylvian) 

 and lateral (intraparietal) sulci appear to become confluent, it is 

 a case of the more stable suprasylvian sulcus being mechanically 

 prolonged, to relieve the tension of the expanding cortex, which 

 would otherwise be accommodated by the lateral sulcus rather 

 than a real confluence of the two sulci. In favour of this inter- 

 pretation it is to be noted that a submerged gyrus indicating the 

 site of the supposed union of the lateral and suprasylvian 

 elements never occurs (so far as I am aware), whereas in those 

 cases where two sulci become confluent {e. g. in the case of the 

 calcarine-retrocalcarine junction) a submerged gyrus frequently 

 occurs to indicate the site of the junction. On this hypothesis 

 alone, it seems to me, can we explain the development in such 

 cases of the aborted lateral element (fig. B, si.). 



Although the lower extremity of the suprasylvian sulcus does 

 not emerge from the Sylvian complex on the surface of the brain 

 (compare Lemur, Perodicticus, inter alia), the separation of the lips 

 of the " Sylvian fissure " reveals the fact that it is composed of two 

 distinct sulci (suprasylvian and pseudosylvian) bounding a tri- 

 angular submerged area, as in the genus Lemur (Mem. fig. 6, a). 

 It is of interest to note, however, that the opercular anterior lip 

 of the suprasylvian sulcus is relatively greater {i. e. more pithe- 

 coid) than it is in the adult Lemur. 



I can find no trace of the rhinal fissure, except on the mesial 

 surface of the hemisphere (fig. C, «), where it presents a form 

 such as we are already familiar with in Lemur (Mem. fig. 5, a). 



There is a typical postsylvian sulcus (Mem. fig. 6, sp.). 



On the mesial surface there is no'rostral nor genual sulcus. 



