BY G. ELLIOT SMITH. 645 



of Eutlieria, regarded the dorsal layer of the marsupial hippo- 

 campal commissure as a true corpus callosum. 



In the descriptions given above, I have descriljed the appearance 

 seen on section through the mesial wall rather than through the 

 middle line, because in the latter plane the fibres have become 

 collected into a more compact mass, and the simplicity of the 

 arrangement is correspondingly obscured. It may be pointed out 

 in this connection that the marked difference in the appearance 

 presented on mesial section of, a.y., such brains as those of Echidna 

 and Macropiis, are more apparent than real. 



The mass of grey matter, which is situated on the ventral aspect 

 of the hippocampal commissure in Onnthorhijnchus and which fills 

 up the concavity of the hook formed by the commissure in 

 Peiaineies, is the homologue of the septum lucidum of Placentalia, 

 being formed, like the latter, from the thickened lamina terminalis. 

 The septum lucidum, therefore, in Marsupials (fig. 1) fills up the 

 concavity of the " psalterial " hook, lying vnitral to the dorsal 

 limb or cornu of the hippocampal commissure, which alone is 

 present in Platypus (fig. 2), dorsal to the ventral limb or cornu, 

 which alone is present in Eutheria, while, as already stated, it lies 

 in the Marsupial between the ventral and dorsal limbs of the 

 commissures, which are both present. The septum lucidum, 

 therefore, presents the same topographical relation to the corpus 

 callosum of placental mammals as it does to the doi'sal limb of the 

 hippocampal commissure of non-placental mammals, so that 

 Flower's* main argument for the existence of a corpus callosum — 

 that it lies dorsal to the septum lucidum — loses all its cogency. 



In this connection it is important to emphasize the fact that 

 the " septum " lies entirely upon the ventral aspect of the hippo- 

 campal formation in Platypus and ventral to its anterior part in 

 Marsupials. It lies, therefore, entirely ventral to the Randbogen, 

 below which it is carried backwards with the superior commissure 

 as the latter develops. Now the corpus callosum occupies the 

 same position in relation to the septum pellucidum as the dorsal 



* Loc. cit. 



