4-04 PliOF. G. ELLIOT SMITH OX THE 



111 the Cliiroptera the suprasylvian and lateral sulci are sometimes present in. the 

 genera Fteropits and Cijnonycteris, and resemble the corresponding furrows in the 

 Edentate Tamandua. 



In the true Insectivora there is usually an orhUal sulcus only. This is particularly 

 well-developed in Gynimira. The little brain of the aberrant Galeopithecus is peculiarly- 

 rich in sulci. It possesses a very dee]} and unusually extensive suprasylvian sulcus of 

 Tinusual form, and in addition well-defined orbital and pseudosylvian sulci. 



The smaller Polyprotodont Marsupials resemble the Insectivora in possessing no well- 

 defined sulcus except the orbital on the outer surface of the cerebral hemisphere. In 

 Tliylacinus and most of the Diprotodontia there are also suprasylvian, pseudosylvian, and 

 prorean sulci ; and in Phascolomys and many of the MacropoJidse there is a typical 

 lateral sulcus *. 



If all of these scattered data be collated it will be possible to determine those features 

 which are the common property of all the Metatheria and of those Eutheria which do 

 not belong to the Order Primates. If we attempt to form a hypothetical type of cerebral 

 hemisphere embodying all these features, it must be macrosmatic and must present a 

 well-defined I'hinal fissure. It must have a deep and well-defined suprasylvian sulcus, a 

 small orbital sulcus which neither joins the riiinul fissure nor is prolonged far in the 

 mesial projection: coronal and lateral sulci which are nearer the sagittal than the 

 transverse direction and which are quite separate. In addition there mat be pseudo- 

 sylvian, postsylvian, postlateral, oblique, ansate, and crucial sulci, for none of these are 

 the exclusive property of one Order, but all, as the brief review given above shows, tend 

 to appear in all mammals. Moreover, there is to be noted a tendency in several Orders 

 (in Myrmecophaga among the Edentates, in Dolichotis among the Hodents, and in the 

 Seals among Carnivora) for the area which separates the overlapping parts of the supra- 

 sylvian and pseudosylvian sulci to become submerged or so reduced that the two furrows 

 form one sulcal complex (fig. 00). 



If, after this review of the conditions which prevail among the Mammalia in general, 

 the plan formed by the sulci in the Lemurs be examined, it will be apparent that the 

 •cirrangement in the latter is unquestionably a slightly modified form of the hypothetical 

 common mammalian type. This exact reproduction of just those sulci which are most 

 stable in other mammals, and the A'ariability shown by just those furrow^s which are 

 less stable in other mammals, can be no mere fortuitous phenomena, but positive 

 ^^vidence of morphological identity. 



There can be no doubt that the Sylvian fissure of the Lemurs is formed by the peculiar 

 union of the suprasylvian sulcus with the less stable pseudosylvian of other mammals. 

 .1 ust as the suprasylvian element in this complex is the only sulcus of the lateral 

 surface which is ab.^olutely constant in all Lemurs, so it is the most stable sulcus in the 

 Mammalia generally. That it is really the suprasylvian sulcus which is found in the 

 Lemurs is abundantly shown by the peculiar condition found in C/uromys. 



In order of constancy the orbital, lateral, and coronal sulci come next, just as also 



* These homologies in the Marsupialia arc suggested by a comparison of the mode of development of ihe furrows 

 ill Ibc brain of J/wrojuti- and that exhiMtcd in other mammals. e-*peeially certain Ungulala. 



