90 University of California Publications in Zoology 
In Lepisosteus platystomus the gland is almost identical in 
form to that in L. osseus, but the ear-like projections are rela- 
tively shorter and stouter than in any specimen of the latter 
species which I have examined. 
The size of the gland as compared with the rest of the brain 
is considerable. The width of the main body in a 25 em. speci- 
men is slightly greater than the width of the optie lobes, which 
form the widest dimensions of the brain, while the width from 
tip to tip of the lateral lobes is more than twice that of the 
cerebrum at its greatest width. Its length is approximately that 
of the cerebellum, and its thickness such that it attains the level 
of the dorsum of the cerebellum. In adults the size of the gland 
as compared with the rest of the brain is even greater than this. 
The entire brain of Lepisosteus is covered by a deeply pig- 
mented membrane which has a peculiar metallic appearance. in 
which the enormous pigment cells appear under the binocular 
microscope as a sprinkling of pepper. This covering membrane 
seems to be directly continuous with the covering of the myelence- 
phalic gland, or with the gland itself, since in microseopie section 
there is no distinctly differentiated outer covering. In other 
words, the structure seems to be a very highly developed and 
enlarged portion of the pigmented covering of the brain. 
HISTOLOGY 
In order to determine the histological structure of the mye- 
lencephalie gland, the best specimen available was fixed in for- 
malin, imbedded in paraffin, and cut in sections 12 microns in 
thickness. The sections were then stained in Delafield’s haemo- 
toxylin, and counter-stained with eosin or with picro-fuchsin. 
When placed under the microscope, it became at once ap- 
parent that the strueture under study was not of nervous tissue, 
but appeared as an enormous development of the pial covering 
of the brain. In some of the sections there appeared to be a 
cavity, but this proved to be nothing more than a very large 
central blood vessel or sinus. In the section drawn (pl. 10, 
fic. 1) the true pia mater is indistinctly differentiated from the 
eland across the dorsum of the fourth ventricle and is fused 
with it. Where the pia mater curves down over the side of the 
