326 



Prof. G. Elliot Smith. 



[Jan. 15, 



diameter, and is placed so obliquely that its surface looks almost 

 directly backward. Shallow but clearly denned furrows separate 

 these two bodies from each other and from the area c. In the artificial 

 cast there is only a very faintly-marked indication of these bodies 

 <fig. % d). 



At a first glance it might seem that they represent the whole cere- 

 bellum, in which case c would be part of the cerebrum ! But careful 

 examination of the natural cast renders such an interpretation highly 

 improbable, and comparison with the artificial cast seems to finally 

 establish the belief that the whole of the region marked c forms part of 

 the cerebellum. 



It is extraordinarily difficult to accurately interpret this peculiar 

 form of cerebellum. A comparison with other primitive types of 

 cerebellum* points to the probability that the lateral buttresses of the 

 mass c represent the floccular lobes, and that the walnut-like mass (d) 

 represents the cerebellar lobule which I have called "area C " (ojj. cit., 

 ' Catalogue,' p. 211). If it be objected that the lateral buttress-like 

 mass is much too extensive to be entirely " floccular," attention may 

 be called to the fact that in the large aquatic Sirenia, which have 

 retained an exceedingly primitive type of brain, the floccular lobes are 

 enormous in comparison with those of other mammals (op. cit., 

 1 Catalogue, 5 p. 346). 



It would perhaps be difficult to find elsewhere in the mammalia a 

 greater contrast than that presented by the smooth, reptilian-like 

 cerebral hemispheres of these casts and the highly complicated, ultra- 

 mammalian neopallium of the recent whales, both Odontoceti and 

 Mystacoceti.f And yet, if we inquire into the nature of the factors 

 which have moulded the form and determined the size of the various 

 parts of the brain in Eocene times and at the present, the contrast 

 between the brain of Zeugloclon and the-modern Cetacea loses much of 

 its significance, and becomes much less peculiar, even though it may 

 not be wholly explained. 



In most Eocene mammals the cerebral hemispheres were exceedingly 

 diminutive in comparison with those of their modern descendants and 

 successors. Moreover, the bulk of the primitive mammalian hemi- 

 sphere was composed of those parts (hippocampus and lobus pyri- 

 formis), which are pre-eminently olfactory : in other words, the neo- 

 pallium (i.e., that part of the pallium which is neither hippocampus 

 nor pyriform lobe) is especially insignificant. It is a well-known fact 

 that the sense of smell loses much of its importance in mammals of 

 aquatic habits (e.g.. Ornithorhynchus, the Sirenia, the Pinnipedia, and 

 especially the Cetacea), and in these animals the olfactory parts of the 



* Compare, for example ( ' Catalogue of the Eoyal College of Surgeons,' 2nd 

 edition, vol. 2, 1902), Armadillo (p. 211), Tapir (p. 311), Manatee (p. 346). 

 t Vide ' Catalogue of the Koyal College of Surgeons,' op. cit., pp. 348 — 359. 



