FOREBRAIN MORPHOLOGY 437 
inverted forebrain, where the medial portions were not joined in 
the mid-line. Such a forebrain is present in the recent Dipnoi. 
Thus it seems very probable that also in primitive crossopter- 
ygians such an inversion was present. This assumption is sup- 
ported by the fact that in the holocephalians and in petromyzonts 
this brain type is the rule. 
In the ependymal roof of the forebrain there is in ganoids and 
teleosts and also in Dipnoi and holocephalians a medial infolding 
in front of the paraphysis. This medial fold might have been 
present also in the primitive crossopterygians. (In holocepha- 
lians this fold begins at the top of the recessus neuroporicus.) 
A paraphysis was also probably present. 
The hemispheres were in primitive crossopterygians probably 
moderately evaginated. This is an assumption based upon the 
fact that in Polypterus, ganoids, and teleosts the evagination is 
confined to the foremost part of the forebrain, the bulbus olfacto- 
rius, and in holocephalians also is not very pronounced. Against 
this assumption is the fact that in recent Dipnoi the hemispheres 
are excessively evaginated. But here the evagination is of a quite 
peculiar nature, being confined to a great extent to subpallial 
brain parts. The evagination, corresponding to that in ganoids, 
that is, the evagination that ends in the bulbus, is not greater, 
however, than in holocephalians. In recent Dipnoi (except in 
Ceratodus) the bulbar ventricle is rudimentary. As a special 
bulbar ventricle is absent in holocephalians, present in Polypterus 
and ganoids and also in selachians and petromyzonts, it seems 
probable that also in primitive crossopterygians there was a 
bulbar ventricle, at least of moderate size. 
In all vertebrates with inverted and evaginated forebrain a 
zona and a sulcus limitans medialis (except in cyclostomes) and a 
sulcus limitans externus occur. These structures might have 
also been present in primitive crossopterygians. As the occur- 
rence of a zona limitans lateralis is variable in vertebrates, it is 
not possible to determine whether such a zona was present in the 
crossopterygians. 
In the subpallial parts, the ventricular position of the nuclei 
in Dipnoi seems to indicate that the same was the case in the 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 5 
