;5rj4 PEOF. G. ELLIOT SMITH ON THE 



The rapsial surface of tlie hemisphere in the Indrisinte is known to me only in the 

 drawings (Hist. Madag-. (2) tome vi. pi. 86. fig. G, pi. 87. figs. 2 b and 4 c) of Milne- 

 JMwards's monograph. It closely resembles that of Chiromys, which will be described 

 later on in these notes. 



Nothing is known of tlie smaller brain of Arahis except the scanty inforaiation 

 contained in Milne-Edward.s's memoir. The plan of the ridges on the interior of a 

 cranium oi Aral/ is laniger (in the Koyal College of Surgeons) is shown in fig. 31. 



Tliere are well-defined representatives of the Sylvian, postsylviau, and orbital sulci, 

 shallow lateral and postlateral furrows, and also a deep sulcus x (in apparent continuity 

 Avith the lateral sulcus) rimning parallel to the Sylvian fissiu-e. Comparison with the 

 brain of Iiidris (fig. 30) seems to point to an homology with the element I'" in that genus. 

 Comparison with the plan found in Fropithecus (compare fig. 24) seems to suggest its 

 identity with the sulcus/of that genus. In Milne-Edwards's specimen it is not so distinctly 

 transverse in direction. In my specimen there is no trace of any ridge corresiwndiag to 

 the coronal sulcus. All this variability of the furrows around the adjacent ends of the 

 lateral and coronal sulci in the Indrisinae shows how unstable this region is in these 

 miimals. This instability is probably due to two distinct factors. In the first place, this 

 area is the seat of very pronounced growth-changes in all the Primates, and especially 

 the lowlier members of tlie Order, because the excitable area of the cortex is expanding 

 to a much greater extent than in other mammals. At the same time, there is in tlie 

 Ijemnrs, and especially the Indrisina?, another and a greater disturbing factor, which is 

 the pionouuced tendency to a disintegration of composite sulci : this must be the 

 expression of some retrogressive process of evolution. Tliis variability is perhaps best 

 exemplified by the behaviour of the sulcus /', wliich seems to be linked in different 

 individuals of the Indrisinse to the coronal sulcus, the sulcus c, and even to the lateral 

 sulcus. 



The Cerebral Hemispheres in the Genus Datjbentonia 



[more commonhj called Chiromys). 



The brain of the Aye-aye {Baubentonia mcuhfgascariensis, Geoff., Chiromys madagas- 

 cariensis, Cuv.) is of extraordinary interest to the morphologist by reason of the 

 possession of what Chudzinski calls " contradictory characters." The same writer 

 lurther explains this remark by the statement that although the Aye-aye is no larger 

 than a small rabbit, or even not so big, it has a most voluminous brain, very rich 

 in cerebral folds*. He adds that "the form of the l)rain is ovoid, but its anterior 

 extremity is mai'kedly i-ounded. In this respect it resembles the l)raiu of certain 

 ]lodentia (Castor, Ilijstrix) and even certain Marsiqiials {Pl/ascolomi/s)." Although 

 these comparisons are quite justified, rej^emhlances of a closer character are to be found 

 within the Prosimise themselves. Gervais describes the crucial cast of the Aye-aye as 

 being "plus arrondi et plus bouibe a sa face superieure que celui des Makis, du 

 Propitheque etde ITndri " f. He compares the appearance of its upper surface with that 

 of the Carnivore Ail uriis fid gens. 



* Hull. Sw. Antlirop. svr. 4, vii. 1S9G, p. i:!. t Jourii dc Zool. i. 167l\ p. 23. 



