300 PEOF. G. ELLIOT SMITH OX THE 



liis specimen the / clement is a larger transverse sulcus and the e element is a small 

 Y-shaped furrow, which might possibly be explained by the condition seen in the left 

 hemisphere of my specimen a. 



On both sides of three crania which I examined there is a long ridge representing a 

 fairly extensive *' central sulcus," such as might be produced by a deepening of the 

 connection between the furrows e and/ on the left side of my specimen /3. 



If we summarize all this puzzling variety in the arrangement of the furrows on the 

 outer surface of the hemisphere of Chtromijs, the great outstanding fact is revealed that, 

 amidst all the variations which so modify as almost to disguise the identity of most of 

 tlie sulci, the suprasylvian and lateral sulci alone remain unchanged. They are the 

 deepest and most stable sulci on the outer surface of the hemisphere. 



The other feature of interest is the strongly marked tendency toward the formation 

 of a transverse sulcus from the elements e and /, which is either the homologue of the 

 central sulcus or, at least, in the present state of our knowledge, fulfils all the known 

 criteria of a sulcus of llulando. 



The peculiar variations which have been noted in the cerebral hemisphere of Chiromys 

 can only be explained on the supposition that a large and highly convoluted brain is 

 undergoing a process of retrograde metamorphosis which leads to the disintegration of 

 all except the most stable sulci. It is altogether inconceivable that the unstable group 

 of fragmentary sulci which rej^resent the pseudosylvian, postsylvian, and coronal sulci 

 can be stages in the evolution of a more complex pattern of furrows. All these facts 

 point to the conclusion that Cliii'omijs is not the lowly Carnivore-like primitive Lemur 

 which most other writers on cerebral anatomy suppose, but a highly-specialized Pi'imate 

 which has undergone most pronounced retrogressive changes, as the result of wdiich all 

 the more variable features of its brain stand revealed and, by contrast, show the underlying 

 common mammalian featiu'cs, which are the heritage of all ttie Meta- and Eutheria. 



None of the writers who have described the brain of CItiromijs has placed on record any 

 information concerning the mesial surface of the hemisphere. I have examined this 

 aspect in four examples. 



The corjDus callosum is 23 mm. long and very plump, Avith a considerably thickened 

 splenium. Its anterior part is bent downward so as to be boomerang-shaped (fig. 37). 

 This is unlike that of the other Lemurs, and may possibly be due to a retraction of the 

 anterior regions of the hemisphere, when the latter lost the more pointed form which it 

 must have had in the immediate ancestors of Chiromys. For the peculiar blunt anterior 

 pole of the hemisphere cannot be primitive, and when this process took place it probably 

 induenced the form of the corpus callosum on the mesial surface, just as it has modified 

 the coronal sulcus on the lateral aspect of the hemisphere. 



The hippocamjjus is distinguished by the possession of an unusually large hippocampal 

 tubercle (fig. 38), and also possesses that peculiar furrow {a) which we have seen ia the 

 genus Lemur. I know of no brain other than tliose of the Primates in which these two 

 peculiarities are both found. The particular form they assume in Chiromys is distinctly 

 Prosimian. 



Tlie most interesting featiire on tlie mesial surfiice is the grouping of the sulci. 



