436 NILS HOLMGREN 
the brain in the living descendants, it seems possible to get a 
rough idea of this subject, especially if the phylogenetic relations 
of the extinet groups are used for guidance. 
From the crossopterygians the Dipnoi, the polypterids, and the 
actinopterygians are generally derived. Of the crossopterygian 
descendants, the Dipnoi have an inverted forebrain, the polyp- 
terids and actinopterygians an everted. Assuming that the 
descent of these three groups of fishes is rightly interpreted by 
paleontologists, the eversion took place in the crossopterygian 
group or in nearly allied descendant groups. ‘The crossopterygians 
are found from the lower Devonian to the upper Cretaceous. 
But the actinopterygians began also in the Devonian with crossop- 
terygian-like fishes, as the palaeoniscids and others. Thus 
the eversion of the forebrain may have taken place very early, 
probably already in Devonian fishes, or perhaps earlier.? 
From primitive crossopterygians the Dipnoi undoubtedly 
are descendants. The earliest Dipnoi are found in the Devonian. 
In the living species the forebrain is inverted. Thus it seems to 
be probable that the inverted forebrain, as being the primitive 
type of vertebrate forebrain, also was the brain type in primitive 
crossopterygians of the Devonian. As in all recent descendants 
of the crossopterygian group and also in the plagiostomes the 
pallium contains two cell-layers, a cortical and a ventricular, this 
must also have been the case in the crossopterygians. Further, 
it is probable that the pallium was already subdivided into the 
typical three parts, the hippocampal, the general, and the pyri- 
form pallium. ‘This is a conclusion drawn from the fact that in 
Polypterus, ganoids and teleosts the subdivided pallium is the 
rule. But also in Dipnoi there are signs of a subdivided pallium. 
Also in the parallel group of selachians a similar subdivision is 
present, at least in the embryo. The eversion of the forebrain in 
Polypterus and ganoids must have taken its origin from a slightly 
7 Judging from the shape of the skull and the position of the optic and troch- 
learis foramen in the newly described crossopterygian genus Wimania from the 
Triassic, the forebrain ought to have been very elongate and laterally compressed 
as in dipnoans and Polypterus. In the triassic palaeoniscid Birgeria the brain- 
ease is much shorter, laterally compressed and rapidly narrowed in front, sug- 
gesting a forebrain of the same type as in the young Lepidosteus. 
