420 NILS HOLMGREN 
the fimbrial portion of the septum. This portion, the antero- 
dorsal, is filled up by more or less scattered cells derived probably 
from the ventricular wall of the fimbrial portion. These cells 
represent probably the nucleus lateralis septi. In the postero- 
ventral part of the septum the medial border of the tuberculum 
olfactorium is situated. This border is continued dorsad by a 
lamina of scattered small cell-groups, representing probably a 
nucleus medialis septi. 
The dipnoan forebrain compared with the selachian 
At first sight the forebrain of Protopterus seems not to have 
many features reminding one of that of Acanthias. Nevertheless, 
a closer examination will reveal dispositions recalling very strongly 
the selachian conditions. In the subpallial parts the great 
extension of the tuberculum olfactorium is common to the two 
groups. Also the extension of the lateral olfactory nucleus, as 
determined above, seems to be in principle the same, the apparent 
difference being caused by the greater differentiation in selachians. 
The septal portion of the subpallium is different, the fimbrial 
portion being much greater in Protopterus than in selachians. 
But there is, on the other hand, a great resemblance in conditions. 
A sulcus septalis occurs in both brains and the fimbrial portion 
does not accompany the medial border of the hippocampal 
formation dorsal to the foramen monroi as in higher vertebrates. 
In selachians, where the hemispheres are joined in the middle 
line, there is no tela fastened at the posterior margin of the fim- 
brial portion of the septum as in Protopterus, where the hemi- 
spheres are not joined. This tela in Protopterus is probably a 
part of the lamina (terminalis or) supraneuroporica, which has 
come to have its attachment at the fimbria owing to a secondary 
displacement of the nucleus lateralis septi, a displacement which 
is not difficult to follow in the form-series: Chimaera, Acanthias, 
Protopterus and Rana. 
The pallial parts of the forebrain seem to be most difficult to 
compare in the dipnoan and the selachian brain. A comparison 
between the adult Acanthias brain and that of a Protopterus is 
