THE FOREBRAIN OF THE ALLIGATOR 355 
than as a receptive center, its main incoming impulses, so far as 
known, coming in through the alveus. As Elliot Smith and 
Levi have suggested, this is probably the forerunner of the hip- 
pocampal cortex as distinguished from the gyrus dentatus. 
Johnston (15), however, regards this dorsal cortex as the fore- 
runner of the mammalian subiculum. From the region of the 
infolding of the primordial general cortex (fig. 6) in the alligator 
this dorsal part of the hippocampal cortex is continuous with the 
general cortex. 
The presence in the alligator of a primordium hippocampi, 
such as Johnston (’13 and 715) has described in turtles, has al- 
ready been mentioned. In the turtle that author has shown the 
presence, on the medial wall of a fimbrio-dentate sulcus (Elliot 
Smith’s suleus limitans hippocampi) between the dorso-medial 
portion of the hippocampus and the primordium hippocampi 
and a sulcus limitans hippocampi and a cell free zone between 
the primordium hippocampi and the parolfactory area (Her- 
rick’s ('10) septal area) in the anterior part of the brain. On 
the ventricular side of the medial wall in the turtle are two sulci 
which correspond to those on the lateral wall and separate the 
same areas. In the alligator in the material which was studied, 
no well defined sulci are in evidence on the medial surface of the 
hemisphere in these regions but the ventricular sulci are present in 
positions corresponding to those in which they are found in the tur- 
tle; and the primordium hippocampi, in the more anterior part of 
the hemisphere, is separated from the septal or parolfactory area 
by a cell free zone. An interesting fact, but one whose significance 
is not clearly understood, is the presence on the ventricular surface 
of a relatively thin ependymal layer over the dorso-medial por- 
tion of the hippocampus and of the primordium hippocampi, 
which becomes thickened over the septal or parolfactory region. 
Under the head of the parolfactory area, the relative positions 
and the relations of the primordium hippocampi and the lateral 
parolfactory nucleus have been discussed. 
Johnston (13, figs. 23 to 27, pp. 446-447) has shown that the 
primordium hippocampi extends forward in the hemisphere con- 
siderably anterior to the hippocampus proper. In the alligator 
