328 



Prof. G. Elliot Smith. 



[Jan. 15, 



to that of the cerebrum. In the case of Glyptodon I four years ago 

 attempted to explain these facts in this manner. 



The development of the neopallium in mammals opens up the 

 possibility of the performance of many more complex muscular acts 

 than are possible in the Amphibia or Reptilia : these acts require 

 a co-ordinating mechanism, the size of which will be largely determined 

 by the bulk of the muscular masses, the actions of which are to be 

 harmonised, and the extent of the sensory surfaces which send into 

 the cerebellum streams of controlling impulses. A large cerebellum 

 is being demanded by a large mammalian body, even if the cerebrum 

 is small. I cannot offer any more satisfactory explanation of the 

 magnitude of the cerebellum in Zeuglodon than this. 



It is clear from the foregoing that the extraordinarily great contrast 

 in the appearance of the brain of the Archseoceti and that of the 

 Cetacea cannot be urged as a reason against their kinship, when it is 

 remembered that the operation of known factors is quite sufficient 

 to explain the transformation of the one type into the other in the 

 time which has separated the Eocene period from the present. 



Having disposed of these negative arguments, we may consider the 

 positive evidence for Cetacean affinity in the brain of Zeuglodon. 



The shape of the cerebrum, and especially its relatively great 

 breadth, is peculiar. In fact, this form of hemisphere rarely or never 

 occurs among mammals, other than the Cetacea. I have elsewhere* 

 attempted to explain the shortness of the Cetacean hemispheres 

 by the fact that the abortion of the basal (olfactory) parts of the 

 cerebrum limits their longitudinal extension. This, however, is not 

 the whole explanation, because in many microsmatic Sirenia (Halicore), 

 and Pinnipedia (Otaria, Phoca) the hemispheres are not especially 

 broad. The disproportionate breadth seems, in fact, to be to some 

 extent a characteristic of the Cetacea ; and, in this respect, Zeuglodon 

 agrees with them. 



The peculiar elongation of the olfactory peduncles beyond the 

 anterior extremities of the hemispheres is rarely found in mammals, 

 though it is common enough in Reptiles and the Ichthyopsida. 

 In fact, the exact parallel to the condition found in Zeuglodon occurs 

 among recent mammals only in the Cetacea. f An analogous condition 

 is found in the extinct Lemuroid Megaladapis [described by Forsyth 

 Major (pp. tit.)] and some Amblyopoda. 



It is not without interest to note that the two outstanding features 

 of the cerebral hemispheres of the Archseoceti, even if their value as 

 indices of kinship be slight, both find their nearest parallel in the 



* 'Catalogue of the College of Surgeons,' op. cit., p. 350. 



f Full references to this are given by Forsyth Major, "On the Brains of Two 

 Sub-fossil Malagasy Lemuroids," ' Roy. Soc. Proc.,' vol. 62, 1897, p. 48, second 

 footnote. 



