586 



Macropus , Aepyrymtzus, and Phascolarctus\ but. like Flower and 

 Symington before him, he did not venture on any explanation of them. 

 The investigations for my memoir of 1894 were carried out chiefly 

 on the brains of Ornithorhynclms^ Perameles, T'richosurus, and Macropus. 

 In the Monotreme and the smaller Marsupial [Perameles] the common 

 Mammalian relationship of the ventral commissure to the external 

 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 in- 

 creased 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 



Fig. 3. Thylacimis cynocephalus. The mesial aspect of the right cerebral hemisphere. 



Nat. size. 



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 tbe 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^ Halma- 

 turus, Hypsiprymnus^ Dendrolagus^ Trichosurus^ Petaurus^ Pliasco- 

 larctus, 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, SarcopJiilus , Dasyurus, Smmthopsis, Didelphys, 



