MORniOLOGY OF THE BRAIN I\ THE MAMMALIA. 423 



that in the young Pi-opitheexs Edwardsi tho caudal extonsion of tlic cerebral liemispliere 

 over tlic cerebellum is distinctly greater than it is iu the adult. Trom an e.^amiuatiou 

 of the cranium of a young (15 days old) PropithecuH corouatus, which Dr. Forsyth 

 Major lent me, I have been able to establish ibis also for aiiother species of a later age. 

 However, in the brain of a young (15 days old) Lemur fulvii.s; which Captain Stanley 

 Flower gave me, the cerebellum was exposed to as great an extent as in the adult. 

 In different specimens of adult Lemurs, even of the same species, I liave found 

 considerable variety as regards the amount of cerebellum uncovered. 



If we consider the marked disproportion in the size of the brain iu the Lemurs and 

 the Apes respectively, the possibility presents itself that the former may at one time 

 have been much larger than it is now. If such were tho case, it is certain that th(? 

 process of reduction in size would be chiefly expressed in a curtailment of the caudal 

 (occipital) prolongation. 



A study of the peculiar series of variations in the Indrisina?, Cliii-oniijs, and to a less 

 extent in all other Lemurs, clearly siiows tliat the hypothesis of a retrogressive change 

 can alone explain this extreme variability. 



It may be urged that it is not the mere retraction, so to speak, of the caiulal pole of 

 the hemisphere whicli distinguishes the occipital region of the Lemurs from that of tlie 

 Apes, but also the absence of the posterior cornu in the former. This is a feature the 

 importance of which has been grossly exaggerated. It is merely one of the expressions 

 of a large neopallium, and the question of the presence or absence of the calcar and 

 calcarine sulcus is not in any way involved in the issue. In any large mammal (large 

 mammals being provided with actually, though not relatively, more extensive neopallia 

 than the small) the increase in size of the neopallium may lead to the formation of 

 a posterior cornu. We see this in the Carnivora, Ungulata, and Cetacea, as well as iu 

 the Primates. The Primates, being preeminent in neojmllial greatness, naturally present 

 in most cases a distinct posterior cornu. 



The direct result of the "retraction" of the occipital extension of th(! hemisphere 

 would be the obliteration of the posterior cornu. 



The brain of Tarshts, which in most respects is so eminently Propitliecoid, is dis- 

 tinguished from other Lemurs by the possession of a distinct ijosterior cornu indis- 

 tinguishable Irom that of the smallest New- World Apes. 



When it is recalled that the Eocene Lemuroids jiossessed brains no smalhn- in size 

 than those of the recent lorms (Cope), it would indeed be surprising if the ])raiu should 

 have remained stationary in size through all the succeeding ages. 



So far as the brain is concerned, the Galagimc are (with the exception of Tarsias) the 

 most generalized of all the Lemurs. And, in view of the foregoing suggestions, it is not 

 without significance that they also possess the most pronounced occipital prolongation. 



The cerebral features of tlie members of the genera Lemur and Hapalemur are not 

 far removed from those of Galago. The peculiar variability in the extent of cerebelhuii 

 covered by the cerebrum is suggestive of a " retraction " of the latter. 



Far more specialized than these genera are the Indrisina', Lcptdolemui- being in most 

 respects intermediate between the latter and Lciinir. In different modes the Indrisine 



SKCOND SEllIKS. — ZOOLOGY, VOL. VIII. 01 



