228 



Prof. G. E. Smith. On a Peculiarity of the [Mar. 5, 



capsule was found to obtain, but in the two larger Marsupials some 

 fibres of the ventral commissure were found to pursue the aberrant 

 course indicated above. It was perhaps not unnatural to suppose (as I 

 did in that early attempt at interpreting this peculiarity) that the 

 increased size of the neopallium in Trichosurus and Macropus was 

 wholly responsible for the presence of this aberrant bundle. For it 

 seemed that since the commissural fibres of the neopallium had become 

 too abundant to be wholly accommodated by the path provided by the 

 external capsule, they, so to speak, had overflowed into the internal 

 capsular route. 



Upon examining a much larger series of Marsupials than were 

 available when my memoir of 1894 was written, I soon became con- 

 vinced that the explanation of the causation of this peculiarity which 

 I then suggested could not be regarded as alone sufficient. I found 

 the aberrant bundle in all members of the genera Macropus, Halmaturus, 

 Hypsiprymnus, Dendrolagus, Trichosurus, Petaurus, Phascolarctus, and 

 Phascolomys, quite irrespective of the size of the brain and of the 

 extent of the neopallium. On the other hand, I sought in vain for 

 it in Perameles, Sarcophilus, Dasyurus, Sminthopsis, Didelphys, Myrme- 

 cobius, and Notoryctes, even though many of these genera possess larger 

 brains than some of the Diprotodonts. 



These facts seemed to suggest that the aberrant bundle was essentially a 

 distinctive feature of the Diprotodont Marsupials, and it appeared to me 

 that the crucial test of this hypothesis would be afforded by the 

 examination of the brain of Thylacinus, which, although that of a 

 Polyprotodont, is almost, if not quite, as large as the brain of the 

 largest Macropod, and considerably larger than those of all other 

 living Diprotodonts. I accordingly submitted the cerebrum of Thyla- 

 cinus to the test, and found no trace of the aberrant bundle (figs. 3 and 

 4), wherefore it is clear that the presence of this aberrant fasciculus of the ven- 

 tral commissure is distinctive of the Diprotodont ia. 



If we compare the brain of the Diprotodontia with that of the 

 other three Mammalian groups : Monotremata, Polyprotodontia, and 

 Eutheria, the meaning of the aberrant bundle becomes, I believe, 

 fairly obvious. 



A study of the structure of the brain in the Monotremes and the 

 Polyprotodont Marsupials shows that in the progenitor of the Mam- 

 malia all the commissural fibres of the neopallium must have passed into 

 the ventral commissure via the external capsule (fig. 4). 



The most pronounced growth tendency in the earliest Mammals 

 must have been the enormous increase of the extent of the neopallium, 

 for while at the beginning of the Eocene period this was almost as 

 insignificant as it is in the Eeptilia, in most recent Mammals it attains 

 a bulk which far exceeds that of the whole of the rest of the nervous 

 system. This sudden expanse of the neopallium would lead to the 



