THE DIENCEPHALIC FLOOR 229 
project into the corresponding diverticula sacci vasculosi (6). 
These tubular canals open ventrally into a larger subdivision 
of the recessus processi infundibuli which is in relation to the 
pituitary surface of the infundibular process, the hypophyseal 
recess (10). Laterally the hypophyseal recess may be traced 
into the two tapering lateral processes (16) of the infundibular 
process. The post-infundibular recess (35) communicates with 
the post-chiasmatic recess in. a position slightly dorsal to the 
infundibular canal. In the fowl the mammillary bodies (27) 
are partly concealed by the post-infundibular eminence but 
appear as slight elevations in the floor dorso-lateral to this struc- 
ture. Because of this relation they seem to be situated at some 
little distance from the median line on either side having the 
post-infundibular eminence and its recess interposed between 
them. They contain no mammillary recess in the fowl. In 
the bittern the mammillary bodies are dorso-caudad to the post- 
infundibular eminence and occupy a position much nearer to 
the median line than in the fowl. A small mammillary recess 
extends for a short distance into the mammillary body in this 
forme (fig. 5)! 
THE DIENCEPHALIC FLOOR OF THE ADULT SELACHIAN 
(MUSTELUS LAEVIS) 
The mesial view of a model reconstructed to show the left 
half of the interbrain floor in the adult Mustelus laevis is re- 
produced in figure 6. Of the elements entering into the floor 
the lamina terminalis (20) is the most cephalic. The optic 
chiasm (4) is caudal to the supraoptic crest (47) which extends 
transversely across the dorso-cephalic surface of the chiasm, 
and may be followed for a short distance out upon the optic 
nerve. Caudal to the chiasm is the post-chiasmatic eminence 
which differs in certain particulars from the corresponding 
region of the mammal and sauropsid, although it occupies the 
same topographical position. The chief points of difference 
arise from the facts that the three divisions of the post-chias- 
matic eminence in the dog-fish are more pronounced than in the 
