82 FORMATION OF SHELLS OF ANIMALS, ETC., 
Polarized light is indispensable im the examination of 
these objects, for in some instances, as in the young 
hermit crab, at the part where the calcareous and mem- 
branous portions of the shell are continuous, the circular 
forms of globular carbonate are so delicate that no evi- 
dence whatever of its presence can be detected under the 
most powerful lenses, and with the best illumination, but, 
with polarized light it is brought out most distinctly. 
The relative position of the molecular, globular, and lami- 
nated forms of this substance, can be well seen in vertical 
sections of the shell of the ordinary sized crab and lobster. 
In the crab the part of the shell which supports the nippers, 
and in the lobster the claws, answer very well for this purpose. 
But in grinding these parts great care must be taken not 
to remove the deep layer, which being the softest is gene- 
rally the first which is ground off. In the sections which 
I have examined, where this caution was not attended to, 
all the globular layer has been removed. On examining 
with the naked eye the internal surface of the portion of 
the shell of the crab just mentioned, two parts are to be 
noticed, a smooth and a rough one. In the former the 
globular carbonate is seen in the thin vertical sections, 
chiefly im one deep layer of nearly the same thickness. 
In the latter, portions of globular carbonate pass at 
certain points through the entire thickness of the shell, 
from the deep to the superficial surface. Sections should be 
made of both of these, and ground down with the precau- 
tion above mentioned, and afterwards put up in glycerine 
or Canada balsam. There are in all shells whose thickness 
is considerable irregular passages, extending from the 
deepest membranous layer to the free surface of the shell. 
These passages are best seen in vertical sections of decal- 
cified shells, im consequence of being obscured in the 
a i Ce oman 
