394 PROF. G. ELLIOT SMITH OX THE 



angle, Avhioh iwints toward the mesial surface, and so affects the neighbouring area that 

 a vertical sulcus usually develops in response to this mechanical bias. 



The paracalcarine sulcus in the Lemurs obviously represents the ventral element ouly, 

 and not the whole of the parieto-occipital. 



I have introduced the term " intercalary "' to distinguish a furrow which has hitherto 

 received no exclusive title. In most mammals it is joined to the calcarine, and the 

 comjilex is called "splenial": in the Primates it is separated from the calcarine and 

 joined to the genual sulcus to form the calioso-marginal sulcus. In many Carnivores ir, 

 is also joined to the crucial sulcus. It is therefore intercalated between the three 

 furrows — calcarine, genual, and crucial — to any or all of which it may be joined. lu 

 spite of its constancy in the Mammalia, the intercalary furrow is morphologically 

 unstable and readily becomes broken up into several fragments in the larger Ungulatcis 

 and Carnivores. A careful examination of the behaviour of this furrow in the whole 

 Mammalia leads me to regard the paracalcarine as being equivalent to the separated 

 posterior fragment of the intercalary sulcus, or as a new compensatory-calcarine furrow 

 developed only in the Primates. In the case of sulci which possess so little individuality 

 and morphological value as the intercalary and the parieto-occipital, it is of little use 

 attempting to establish any exact correspondence, because this can hardly be said to 

 exist even in two hemispheres of the same brain. All that can be said with certainty is 

 that the tension of the growing cortex, which v/ould be relieved in the Carnivora by the 

 conOuence of the intercalary and calcarine sulci, is instrumental in forming the para- 

 calcarine sulcus in the Lemurs. This does not, however, imjily the identity of the 

 two furrows. The instability of the paracalcarine element is shown in the Cebidai : 

 in CalUthrix it is either absent or very diminutive, in Chrysothrix it forms the typically 

 vertical ventral element of the parieto-occipital sulcus, and in Hf^yctipilhecus it is 

 horizontal. Of course the latter cannot be described as homologous with the typical 

 furroAV of ChrysothrLv, but merely as the expression of the same causal factors. There 

 can therefore be no strict homology between such an unstable furrow and any sulcus, 

 and especially such a variable element as the intercalary, in otlier mammals. 



When we consider how tickle this sulcus is, the fact of its extreme constancy of form 

 and relations in all the Lemurs, not excepting even the aberrant Tarsius and Chlvomys, 

 becomes a valuable testimony to the closeness of the bond of affinity whicli unites all the 

 Prosimite. In fact, a study of the mesial aspect of the hemisphere affords convinciug 

 proof of this undoubted kinship. 



In many Lemurs of various genera an oblique sulcus {b) appears on the ventral side 

 of th(! retrocalcarine sulcvis. A comparison of this with a series of Primate brains shows 

 it to be the posterior part of that composite and morphologically unimportant furrow 

 which is called " collateral " in the brain of Man and the Apes. 



