438 NILS HOLMGREN 
Crossopterygil. But, on the other hand, it must be borne in 
mind that the nucleus olfactorius lateralis and the nucleus taeniae 
in ganoids and teleosts are free from the ventricular ependyma. 
In Dipnoi the nucleus taeniae is also removed from the ependyma 
and the tuberculum olfactorium in its caudal part has wandered 
in from the ventricular wall. Further, the tuberculum olfac- 
torium in selachians and holocephalians, as well as in reptiles 
and mammals, is removed from the ependyma. Therefore, Iam 
disposed to believe that the primitive crossopterygians had a 
tuberculum olfactorium removed from the ventricular wall. 
In summary, this hypothetical crossopterygian forebrain resem- 
bles the holocephalian, if the great secondary prolongation of the 
telencephalon medium of the latter and the subdivision of the 
pallium of the former are disregarded. 
The forebrain of the common ancestors of selachians and 
holocephalians 
The holocephalians probably are a very old group of fishes. The 
earliest representatives are found in Jurassic layers, but the group 
might have been much older. The holocephalians undoubtedly 
are related to the selachians and the holocephalian stem is prob- 
ably a very early branch from the selachian. Selachians are 
known from the Lower Devonian period. 
Assuming that the holocephalians and selachians are descendants 
from a common ancestral vertebrate type, the forebrain of the 
type was an inverted and evaginated one. In selachians the 
inversion is very pronounced, and the halves of the pallium are 
joined in the mid-line almost throughout their whole length. 
This is a condition that must be regarded as due to secondary 
fusion. The evaginated frontal parts or the forebrain also are 
secondarily joined to form the massive septum. In seeking the 
ancestral selachian type of forebrain, these secondary characters 
must be removed. Taking away these characters, the selachian 
forebrain will be reduced to a more or less holocephalian type with 
lateral bulbi olfactorii and subdivided pallium. 
The lateral position of the bulbi probably is primitive. This 
assumption is based on the fact that the bulbar formation in the 
