PARIETAL REGION IN THE PRIxMATE BRAIN 303 



Hapalidae and the lowest of the Cebidae such as Callithrix and 

 Aotus and mconstant in such forms as Chrysothrix and Pithecia, 

 it shows its greatest variation in Cebus. Here it may be repre- 

 sented by a relatively short shallow furrow which is not unlike its 

 first appearing homologue in the Lemurs, or it may be deeply 

 operculated and located considerably farther forw^ard. In the 

 Cercopithecidae it is always well developed. The relation of the 

 free edge of the operculum to the interparietal and parieto-occip- 

 ital will vary in accordance with the degree of operculation, 

 whereas the bottom of the lunate sulcus is much more constant. 

 The sulcus lunatus, as outlined by the anterior edge of the oper- 

 culum, begins near the mesial border of the hemisphere, overlap- 

 ping it in some cases, extends outward and slightly forward and 

 describes a gentle curve whose convexity is directed forward. It 

 stops some little distance above the inferior margin of the hemi- 

 sphere, owing to the interposition of the inferior occipital sulcus. 

 The relief of the anterior wall has ah'eady been mentioned and 

 the posterior fits into this more or less closely. In most cases 

 there are submerged beneath the operculum the parieto-occipital 

 fossa and the termination of the interparietal sulcus and the cor- 

 responding first and second annectant gyri, or even additional 

 gyri, which structures will become superficial with the recession 

 of the operculum. 



Regarding the phylogeny of the lunate sulcus we may quote 

 from Elliot Smith's (I.e., '08, p. 167) writings on the extinct 

 Primate, Lemur jullyi: 



There seems to have been in this brain an exceptionally extensive 

 representation of the furrow which in my former memoir I called 'post 

 lateral' — an identification which I have since proved to be absolutely 

 exact, because this furrow, both in the Carnivora (in the brain of which 

 this sulcus was first so called) and in such Prosimiae as Lemur, Nyctice- 

 bus, Perodicticus and Propithecus, forms the cephalic boundary of the 

 visual cortex or area striata. This observation, which has not hitherto 

 been published (except verbally at a meeting of the Anatomical Society 

 on June 1, 1901) has been recently confirmed in the case of Nycticebus 

 by Oskar Vogt. I refer to this apparently irrelevant matter here to 

 emphasize the fact that when the sulcus lunatus ('Affenspalte') first 

 makes its appearance in such lowly Cebidae as Pithecia it presents the 

 same relationship to the area striata as the sulcus postlateralis exhibits 



