On the Brains of two Sub-Fossil Malagasy Lemuroids. 49 



As lias been pointed out in the description of the skull of 2Iegala- 

 dapis* its post-orbital region is remarkably elongate in the lateral 

 parts, in a manner quite unusual amongst Lemuroidea, and for 

 parallels of which we have to look amongst Carnivora, and especially 

 Insectivora (e.g., Centetes). An external and superficial examination 

 of the skull might lead to the belief that this elongation has resulted 

 in an anterior elongation of the brain-cavity as well. But as we 

 have just seen, in Megaladapis the elongation in question is brought 

 about by the development of air sinuses, whilst the cranial cavity is 

 on the contrary shortened, as well as narrowed. 



Although in this skull the sutures are almost entirely obliterated, 

 it is obvious that in the elongation of the lateral parts of the post- 

 orbital region, the orbits and the alisphenoids participate as well as 

 the frontals. This is well shown by the fact that, whilst in 

 Lemurids generally, as well as in monkeys, the passage for the 

 optic nerves from the internal cavity to the orbits, of which we 

 speak as the optic foramen, is a very short one — very oblique in the 

 former, almost parallel in the latter, — we find in Megaladapis that 

 the second pair of nerves traverse a canal of no less than 24*3 mm. 

 length, before appearing at the outer side of the skull, in the orbits. 

 So that, in lieu of a foramen opticum, we have here a canalis opticus. 

 The united foramina rotundum and lacerum anterius form likewise a 

 canal of about 21*5 mm. length. 



When describing the skull of Megaladapis, I endeavoured to show 

 that its peculiar low condition is not primitive, but pseudo-primitive 

 (Fiirbringer), that is to say, that it has been brought about by a 

 " retrogressive evolution," or a retrograde metamorphosis, if the last 

 term be preferred. If any further proof were needed for this asser- 

 tion, it would be furnished by the conformation of the brain, as 

 described above, for I trust that no anatomist will maintain that this 

 was the primitive condition in Lemuroids. It may fairly be predicted 

 that, when we come to know the skulls of very young specimens of 

 Megaladapis, they will show a much closer approach to the ordinary 

 Lemurid type in the conformation of the brain cavity and its walls, 

 and the gap between the young and the adult in this respect will 

 prove to be wider than perhaps in any other known Mammal. How- 

 ever, in the Insectivora and most of all in Centetes, we find also a 

 very great difference between young and adult in the relative size and 

 conformation of the brain (the brain being even absolutely smaller in 

 the old), whilst the least divergence is to be found in Marsupials 

 on the one side, in Man on the other, and this obviously for opposite 

 reasons. 



Apart from what has been pointed out about the analogy of Megal- 



VOL. LXTT. 



* Loc. cit., p. 16. 



E 



