102 FORMATION OF SHELLS OF ANIMALS, ETC., 
form at the same time.) Such specimens can either be 
put up in glycerine or in Canada balsam. And it is 
scarcely necessary to add, that they are only to be met 
with occasionally, and under favorable circumstances, 
when the development of these parts of the shell is just 
in the proper stage. The parts of these shells, on which 
the globular carbonate of lime occurs (though in an in- 
ferior degree of perfectness of form and transparency to 
those just described), to be next examined, are the septa, 
situated between the interlaminar cavities. These cavities 
are always filled with saltish water containing soluble 
animal matter, and free carbonate of soda, so that red- 
dened litmus paper put into this water soon becomes blue. 
The globular deposit is always on the surface of the lamina 
in contact with the fluid, and therefore the lime has most 
probably been furnished by this fluid, after the alkaline 
carbonate, still in excess, had decomposed the lime-salt 
held by this fluid in a state of solution. Hence there is 
in this case very strong evidence that the lime entermg 
into the composition of the globular carbonate comes 
directly from the water in which the animal lives. The 
only objections to this conclusion which can be raised, 
are, that the water in these spaces never was contaminated 
with a salt of lime, or, if it had been, this salt was sepa- 
rated from it before the solution could get access to the 
cavity. Now, as all common water contains lime in solu- 
tion, in one or other state of combination, the first objec- 
tion can have no weight; as to the second objection, it 
may be observed, that the water contained in these spaces 
has lost a substance which it had before held in solution, 
but that it now contains an excess of the very compound 
which always precipitates this substance, a sufficient reason 
for its absence. But, besides, there is animal matter in 
