428 J. B. JOHNSTON 
descent of reptiles and mammals. The dipnoans, or some of 
them, do stand in this line of descent and the evolution of the 
brain must be traced from the selachians, through the dipnoans, 
reptiles and marsupials to the mammals. 
The main purpose of the present paper is merely to define 
the morphological relations of the chief cell masses in the brain 
of the turtle in order that these may be used in future studies 
as points of reference in comparing ichthyopsid, reptilian and 
mammalian brains. This is done in the belief that a satisfactory 
account of the mammalian and human brain must eventually 
include an account of the origin and evolution of the several 
structures together with an explanation of existing structural 
relationships and the course of their evolution on the basis of 
environment, habits of life and the function of the several parts. 
The olfactory area 
Upon comparing the secondary olfactory centers of the hinge 
turtle with those of fishes, the only very noteworthy fact is the 
appearance in the turtle of a nucleus of the lateral olfactory tract 
behind the area occupied by the lateral forebrain bundle. This 
is a distinctly mammalian character. In the selachian (John- 
ston ’11 a) the lateral forebrain bundle is imbedded in. the somatic 
area in. the lateral wall of the slender portion of the forebrain 
known to many authors until recently as the ‘praethalamus,’ and 
the lateral olfactory area lies wholly in front of this. The re- 
lations are shown in figure 59, in which are roughly indicated the 
areas from which alone the pyriform lobe and the nucleus of the 
lateral olfactory tract may have been developed. In the turtle 
these areas have moved far backward on the lateral surface and 
the mass which imbeds the lateral forebrain bundle appears 
as an island surrounded by olfactory nuclei,—the striatal area 
surrounded by the pyriform lobe, nucleus of the lateral olfactory 
tract, the diagonal band, the parolfactory nuclei and the tuber- 
culum. This condition, which is so suggestive of, but not wholly 
similar to, the mammalian condition, has come about by a 
spreading and migration caudad of the lateral olfactory area, 
