440 NILS HOLMGREN 
The absence of a medial infolding of the tela and of a zona 
limitans lateralis may be primitive or secondary; it is not possible 
to decide which of these alternatives is right. 
The situation of the bulbi olfactorii in front probably is 
secondary. The entrance of the olfactory nerves somewhat lat- 
erally and the lateral position of the bulbar formation in the 
embryo seem to indicate this. 
The preceding attempts to deduce the type of the forebrain in 
the hypothetical ancestral types are very uncertain, but the uncer- 
tainty increases very much in making an attempt to reconstruct 
the brain type of the common ancestors of the cyclostomes and 
the other fishes. But it must be borne in mind that the only way 
to get any information of these brains is that based on deduction 
from the recent fauna. 
The brain type, however, of the common fish ancestors may 
have been inverted and at least evaginated in the bulbus olfac- 
torius, which had a lateral position. The pallium was probably 
not subdivided. There was a sulcus limitans lateralis present. 
The subpallial nuclei were not differentiated. 
According to the ideas set forth above, the subdivision of the 
pallium has taken place in a very remote period, probably already 
in the Silurian. In this period also the subpallial nuclei seem to 
have been differentiated and the frontal evagination taken place. 
The inversion of the forebrain is primitive and the Silurian 
fishes probably had an inverted and also evaginated forebrain. 
The eversion, characteristic of Polypterus, ganoids, and teleosts, 
probably took place in the early Devonian period. 
Stockholm, April, 1921. 
