FOREBRAIN MORPHOLOGY 427 
Ganoids and teleosts 
In Polypterus all structures characteristic of all other fishes are 
present. In ganoids and teleosts these structures are repeated, 
but somewhat modified. Thus it is not necessary to describe — 
minutely the brain of those fishes. I can restrict myself to the 
most striking differences. 
In all ganoids the pallium is not of uniform thickness through- 
out, as in Polypterus, and thus the outer surface is not parallel 
with the ventricular. In Acipenser the ventricular surface is 
transversely rather convex, but the outer surface is flat. Thus, in 
a transverse section the distance between the zona limitans and 
the point of attachment of the tela is much longer than the dis- 
tance between the latter point and the fovea externa. In other 
ganoids this difference in length is much greater, as the pallium is 
much thicker in the middle than in Acipenser, and in teleosts the 
tela is attached close to the fovea externa. 
Thus, in actinopterygians (ganoids and teleosts) the develop- 
ment seems to go from a moderately thickened pallium to an 
extremely thickened one; that is, from a pallium with relatively 
long lateral outline to one with very short. 
In all actinopterygians the pallial cells are grouped in three 
more or less well-limited parts, corresponding to the three parts in 
Polypterus. Put in all actinopterygians the pallial cells have 
wandered in from the ventricular sheet to a greater extent than 
in Polypterus. In Acipenser, Scaphirhynchus, and Polyodon the 
cortical cells form cell-laminae with cortical arrangement through- 
out the whole pallium, with but few cells in the ventricular 
position. In a young Lepidosteus (fig. 39) a ventricular layer 
covers the whole ventricular surface, but the main part of the 
pallial cells has wandered in. 
In teleosts the three pallial parts are very well limited, but the 
cortical arrangement of cells is not always well pronounced. In 
Osmerus I have described this arrangement (see my paper in 
Acta Zoologica, vol. 1, 1920). 
The zona limitans is very conspicuous in Lepidosteus (fig. 39, 
z.11.) and Amia (Johnston) and in many bony fishes, but in. 
