376 PEOF. G. ELLIOT SMITH OX THE 



In Levmr (and all the rrosimice) the lateral (anterior) geniculate body becomes much 

 larger and more distinct than it is in most mammals : and this feature it also shares with 

 the other Primates. 



It might perhaps l^c imagined that the larger dimensions and more distinct separation 

 of the lateral geniculate body from the rest of the thalamus are due to the fact that the 

 optic tracts are unusually large in all the Lemurs. But this cannot be the whole 

 explanation, because in many other large-eyed mammals (see many llodents, Ungulates, 

 and others) there is no such distinctness of the external geniculate body as we find in all 

 Lemurs and other Primates. This phenomenon must be due to an increased perfection 

 of tlie cortical path for visual impulses, rather than to a mere enlargement of the lower 

 optic tracts. 



The four quadrigerain-al bodies rise to the same level, and resemble in shape those of 

 the Myrmecophagidte more nearly than those of any otiier mammals (excepting the 

 other Primates). There is a well-defined tractus peduncularis transversus, but it calls 

 for no further mention. 



If the region of the medulla oblongata in the Lemurs be compared with that of other 

 mammals of a corresponding size, the only noteworthy distinctive feature of the former 

 will be the relatively greater prominence of the olivary body. It is, however, somewhat 

 smaller and less prominent than it is in most Apes, the chief olivary nucleus being 

 smaller in proportion to the internal accessory olivary nucleus in the Lemurs than 

 it is in the Apes. 



The Calcakixe Sulcus and the Occipital Prolongation' of the 



Hemisphere. 



The foregoing notes will have made it clear that a triradiate group of three sulci — 

 which I have called calcarine, retrocalcarine, and paracalcarine respectively — form an 

 iirrangement which presents identical feaiures in all the Prosimine. No one has ever 

 questioned the identity of the furrow labelled calcarine with the sulcus so-called in tlie 

 Apes. The sulcus which I have distinguished Ijy the name "paracalcarine" is regarded 

 l)y most writers as the representative of the parieto-occijiital sulcus in the Apes. 

 Plower was, I believe, the first to suggest this view ; but afterwards renounced it in 

 lavour of the hypothesis that the paracalcarine sulcus represents one oL" the limbs of 

 bifurcation which the calcarine sulcus presents in most Apes. When I began writing 

 this memoir I subscribed to the latter view, chiefly because the parieto-occipital sulcus 

 is absent in several of the lowlier Cel^idee and all of the Ilapalid^e ; and it seemed therefore 

 to be a furrow Avhich becomes evolved in the Apes : but in develo2:)ing the argument 

 which is here unfolded, I became convinced that there may be another and more probable 

 Avay of regarding this matter. None of the writers wlio call the paracalcarine sulcus 

 "parieto-occipital" seem to have considered the possibility of it being any other 

 furrow. 



Pv-ecognizing the identity of the calcarine sulcus in all the Primates, we may pass on 

 to the consideration of its relationship to the lateral ventricle, with a view to determine 



