418 NILS HOLMGREN 
little to add to his excellent description. I wish only to point out 
the fact that the pallial formation of Protopterus is still smaller 
than in Lepidosiren. ‘The inversion of the pallium is not great, 
the medial borders of it do not come in contact with each other, 
but are widely separated, especially in the posterior part of the 
forebrain. A zona limitans medialis is present (figs. 29 to 34, 
z.l.m.). On the inside of this zone there is a sulcus limitans medi- 
alis (s.l.m.), occupying the dorsomedial corner of the lateral ven- 
tricle. The sulcus limitans hippocampi (pallii medialis) is con- 
spicuous only in the posterior part of the forebrain a little before 
the foramen monroi (figs. 32, 33, s.l.p.m.). The zona limitans 
lateralis is obsolete, but a sulcus limitans externus is clearly seen. 
At the rostral end of the forebrain this sulcus joins the sulcus 
limitans medialis at the base of the bulbus olfactorius. In the 
pallium a cortical layer (primordial cortex) of scattered cells is 
present (figs. 29 to 34, g.p.c.). This layer passes uninterruptedly 
through the entire pallium. A very distinct medial swelling (as 
in Lepidosiren) of this cortex I have not found; yet there is an 
indistinct thickening at each side of the cortical layer. It is of 
great interest that the ventricular cell-layer at the upper end of 
the ventricle is very thick, protruding against the thinner part 
of the cortex (figs. 29 to 33, g.p.c.) between the two swellings. 
This is a feature tolerably well marked in the photographs of 
Protopterus, as well as of Lepidosiren (Elliot Smith), and reminds 
one of the general pallium rudiment in Acanthias. 
The subpallial parts are extremely developed, as pointed out 
by Elliot Smith. In lateral view the subpallial parts occupy two- 
thirds of the height of the forebrain vesicle, and in medial view 
they cover almost the whole height. The ganglion cells, especially 
in the frontal part of the brain, are ventricular in position (fig. 
sagittal fissure, especially well developed on the ventral side. On the dorsal 
side a sagittal fissure is also present, although of a peculiar nature. The hemi- 
spheres are very strongly evaginated, but the entire dorsal part of the medial 
wall of each hemisphere is made up by a strongly folded ependymal membrane, 
whose folds form a part of the ‘lingula interolfactoria’ of Bing and Burckhardt. 
Thus the forebrain is extremely evaginated, with the foramen monroi situated 
in the hind part of it. A fuller account of the remarkable brain of Ceratodus will 
be given later. 
