IN THE EXTINCT LEMURS OF MADAGASCAR. 167 



together with the very clearly defined sulci emerging from the lower end of the 

 Sylvian fissure, helps to map out the insular area with a distinctness that recalls the 

 condition found in Hylohates and is only rarely met with beyond the limits of the 

 Anthropoid Apes and Hominidse (text-fig. 54). 



There seems to have been in this brain an exceptionally extensive representative of 

 the furrow which in my former memoir (i) I called " postlateral" — an identification 

 which I have since proved to be absolutely exact, because this furrow, both in the 

 Carnivora (in the brain of which this sulcus was first so-called) and in such Prosimise 

 as Lemur, Nycticehm, Perodicticus, and Propifhecus, forms the cephalic boundary of 

 the visual cortex or area striata. This observation, which has not hitherto been 

 published (except verbally at a meeting of the Anatomical Society on June 1st, 1906), 

 has been recently confirmed in the case of Nycticebus by Oskar Vogt (4). I refer to 

 this apparently irrelevant matter here to emphasise the fact that when the sulcus 

 lunatus [" Aftenspalte "] first makes its appearance in such lowly Cebidse as Pithecia 

 (3) it presents the same relationship to the area striata as the sulcus postlateralis 

 exhibits in the Prosimiee — a relationship which undergoes a progressive modification 

 in the higher Cebidse and the Cercopithecidee as the result of the diff'erentiation of the 

 neopallial areas fringing the area striata and the deepening of the sulcus lunatus to 

 accommodate these expanding strips of cortex. In other words, the sulcus lunatus 

 [postlateralis] of the Prosimise, while retaining its similitude to and real identity with 

 the postlateral sulcus of other mammals, presents a much nearer approximation to the 

 condition found in the lowlier Cebidse than the latter presents to the state of affairs 

 met with in, say, Cercopitheciis. 



The transverse form of this postlateral sulcus and its separation from the lateral 

 sulcus (compare i, figs, 1, 2, and 3) seem to occur more often in the species varins 

 than in other living Lemurs. In Lemur jullyi the direction of this furrow, obliquely 

 downward and backward, recalls the form and relationship to the iulraparietal [lateral] 

 met with in the Carnivora and other non-Primate Orders. 



Unlike the brain of Lemur varius (in which the sulci are unusually short, in spite of 

 its large size) in the specimen under consideration all the sulci ever found in any 

 Lemur's brain are developed to their full extent and are deep and cleanly cut. 



The sulcus "/"" of my earlier memoir (i) is exceptionally long and quite sagittal in 

 direction. My recent histological work (and also that of Vogt [4]) and physiological 

 experiments conducted by Dr. Page May and myself [see British Association Report, 

 1904, p. 760] have confirmed the suggestion made by Ziehen and by myself (i, p. 416) 

 that the sulcus "/"" is to be regarded as the upper part of the sulcus centralis. This 

 is only true if we admit certain reservations. The sulcus "_/ " is called into being by 

 the same factors which produce the upper part of the central sulcus ; and in many 

 Prosimise {Perodicticus and sometimes in Nycticebus, Projpithecus, and Chiromys) the 

 resultant furrow is an exact homologue of that sulcus ; but in other cases it often 



