106 FORMATION OF SHELLS OF ANIMALS, ETC., 
polarized light.) I may observe, that, although the 
primary and secondary forms assumed by the globular 
carbonate of lime are perfectly alike in crustaceans and 
molluscs, yet their ultimate forms are very different ; but, 
these differences depend upon mechanical causes, and 
admit of being imitated to a great extent by modifying 
the artificial process. In crustaceans, the ultimate form 
of the carbonate of lime is the same as that of those 
artificial caleuli which are formed on the under surface of 
the glass slide, and which have the laminated structure 
with the radiating lines. These, during their formation, 
have not been so much exposed to pressure from the con- 
tact of the surrounding parts. In the shells of molluscs, as 
the oyster, the ultimate form is that of flattened plates, 
closely connected together, and overlapping one another. 
This form can in some measure be produced in the artificial 
process, by allowing the globular deposit to remain at the 
bottom of the vessel in which it was formed, for several 
months. To produce this effect, the carbonate must not 
be mixed with triple phosphate. The white gum is the 
best in this case, and the quantity of alkali must not be 
sufficient to saturate the thick solution of gum. In good 
natural specimens of this form, globules perfectly spherical 
and beautifully laminated can be seen mixed with those 
in which the process of flattening is gomg on, and can be 
distinguished in its several stages. This is especially 
indicated by the laminated form becoming less and less 
distinct from the circumference to the centre, where 
it remains visible longest, afterwards disappearing alto- 
gether; and the form having become flat, that which was 
a globule then becomes reduced to a very thin plate, with 
an irregular or zigzag margin. During this transforma- 
tion of the globules into plates, and after these plates 
