174 



DR. a. ELLIOT SMITH ON THE BEAIN 



sulcus labelled sco. in my diagram of the brain of Megaladapis (i, fig. 42, p. 365) 

 is also a conjoint orbito-coronal sulcus which has lost its curve and been bent back by 

 the retraction of the frontal pole of the hemisphere, but this seems to be the probable 

 explanation of the curious form of the hemisphere in Megcdadapis. 



In comparison with the other Prosimise, Palceopropithecus has departed least of the 

 three and 3£egaladapis most from the primitive Lemuroid type in respect of this 

 feature. In all three [Palwopropithecus, Chiromys, and Megaladapis) there is a note- 

 worthy reduction in size of the frontal area of the hemisphere. The retraction of the 

 front of cerebrum (which has resulted from this atrophy) has in Palceopropithecus 

 brought the orbital area to the extreme antero-lateral margin of the hemisphere : in 

 Chiromys it has moved a little further on to the pole of the hemisphere ; and, finally, in 

 Megaladnpis the homologue of the orbital surface is probably placed partly on the 

 dorsal aspect of the anterior extremity of the hemisphere. 



Text-fig. 63. 



tefnp. Slip, 

 s. izinai'. 



sirais Lett'- 

 /^ w-^ cercieZhuTi.. 



I s.orb, ', . 



buIb.oT/: I nerr.triffe^L. ^P^' 



s. /hoTzto. orbit. 



Diagram of the left lateral aspect of the brain of PaJaopropiihecus wa.rimits. Natural size. 



This seems to be the outstanding feature of these three brains— a (probably) 

 secondary atrophy of the great frontal association-area, which is the latest part of the 

 cerebral hemisphere to be evolved and the first to go when the brain undergoes a 

 retrograde change. 



The fronto-orbital sulcus, which is absent in both Chiromys and Megaladapis, is 

 present in this brain in the position analogous to that which it occupies in Mesopro- 

 pithecus, Nesopithecus, and Lemur jullyi. 



There is the characteristically small intraparietal [lateral] sulcus of the Indrisinseand 

 (as also in some examples of Propithecus and Indris) the shallow inferior frontal 



