182 
ment of the commissures and the parts they connect in Ornitho- 
rhynchus one would expect the whole of the dorsal commissure of 
Sauropsida to be hippocampal and the parts it connects hippo 
campus. The accounts of Meyer and Edinger of the structure 
of these parts of the reptile brain afford strong confirmatory 
evidence in favor of this presumption; and there does not seem 
to be the slightest ground for the belief that a corpus callosum 
is present in any animal outside the Eutheria. Osborn’s attempt 
(27) to demonstrate this clearly shows the weakness of his case 
by the manner in which he misrepresents the condition of affairs 
in the foetal Macropus, which, as the connecting link in his 
series between the Eutheria (where alone @ priori the presence of 
a corpus callosum is granted) and the submammalia, forms the 
crux of the whole argument. 
It will be noticed from fig. 3 that the fascia dentata is exposed 
in the whole of its extent, and forms a considerable segment of 
the mesial hemisphere wall. In a transverse section through the 
hippocampus in Notoryctes (fig. 6) it will be seen, moreover, that 
the fascia dentata (f.d.) in its whole width forms part of the 
mesial hemisphere wall. In Perameles (fig. 7), although it still 
shares in the formation of the mesial surface, it will be seen that 
a considerable part of it is overlapped by pallium. In Platypus 
in the greater part of its extent (fig. 8 and 36, fig 1), and in 
Macropus (fig. 9) in the whole of its extent, the fascia dentata 
does not form any part of the median surface of the cerebrum, 
being completely hidden by a pallial operculum.* In Petaurus, 
Phalangista, and Potoroo the fascia dentata is hidden throughout. 
In Dasyuwrus and Phascolarctos it is partially exposed, but in 
Perameles and Notoryctes it may be seen throughout. In addi- 
tion to this effect of the increased development of the pallium in 
hiding the hippocampus, it also has an influence upon the mor- 
phology of the region. Thus in no other Mammal does one 
find the simplicity of arrangement which the hippocampus of 
Notoryctes presents (fig. 6), an appearance which recalls the foetal 
hippocampus in Perameles and Macropus. In any non-placental 
Mammal, however, if the hippocampal region be examined at its 
anterior extremity the complexity of the region gradually disap- 
pears by the gradual unravelling of the complicated folds, which 
are seen in figures 7, 8, and 9. In this process of simplification 
a stage may be found in any Metatherian brain closely resem- 
bling that exhibited throughout in Wotoryctes. It may be readily 
seen in any Monotreme or Marsupial brain that the pyramidal 
* Tu all Marsupials and Monotremes the anterior extremity of the fascia 
dentata appears on the surface ; so that the above remarks only apply to 
that part which is placed behind the situation of the hippocampal com- 
missure. 
