56 W. BLAXLAND BENHAM. 



In a specimen (947 /) in our Museum (see fig. 22) the 

 operculum is normally developed. When this is turned back 

 the Affenspalte is exposed, and in front of it the deep parieto- 

 occipital fissure is seen, extending about three quarters of an 

 inch across the brain (fig. 23). On the mesial face this descends 

 for nearly half an inch (fig. 24), but is cut oflF from the rest 

 of the parieto-occipital by a conspicuous gyrus {a.). The main 

 portion of the fissure lies in front of it, and only just cuts the 

 edge of the hemisphere [mes. p. o.). The anterior portion of 

 the fissure {mes. p. o) does not branch. Here, when the 

 operculum is removed, we have the same arrangement as in 

 " Sally." 



In another specimen (fig. 26), in which the edge of the 

 operculum is directed obliquely backwards at its mesiad end, 

 we have a further development of the gyrus intercuneatus, 

 so that the very deep '^ lateral " parieto-occipital [lat. p. o.) 

 scarcely bends round on to the mesial face at all, and lies 

 almost entirely below the operculum, cutting off the intra- 

 parietal from reaching the Aff'enspalte. In fact, it looks at 

 first sight as if it were the " Affenspalte," and as if the gyrus 

 marked a. were the " first annectant gyrus." Such, indeed, it 

 appears to be, for in many cases the " external perpendicular 

 fissure " of authors is not the same fissure as is here called 

 " Afi'enspalte," although it has been homologised with it. 

 The "internal" or mesial parieto-occipital {mes. p. o.) enters 

 the calcarine fissure, the gyrus cunei being, as far as I can see, 

 absent (fig. 27). 



In a brain referred to by Rolleston,^ in which on the right side 

 the operculum is less developed than usual (see fig. 30), the 

 parieto-occipital fissure is very deep, and extends for nearly an 

 inch across the surface of the hemisphere (fig. 32). Here it is 

 limited by a well-defined " annectant " (arcus). Seen from the 

 mesial surface (fig. 21) there is no trace of a division of the 

 fissure into two portions. On both right and left sides there is 

 but a single, nearly vertical cleft — cut off by the gyrus cunei 



1 Rollcston, " Ou the Affinities of the Braiu of the Orang-outang,' 

 ' Nat. Hist. Rev.,' 1861. 



