BY MOLECULAR COALESCENCE. 107 
have acquired their ultimate form, they show no tendency 
to coalesce, but remain intimately connected with the 
membrane on which they were formed. I have several 
specimens showing all the appearances above described. 
There is a form of carbonate of lime occuring chiefly at 
the edge of the valves of these shells, which has been 
particularly noticed by Dr. Carpenter, in the Pinna. It 
also occurs in pearls produced by the oyster, making up 
the principal part of their substance, where, as in other 
parts, 1t can be shown by a proper mode of examination to 
result from the coalescence of other smaller particles into 
globular bodies of various sizes, which occupying the 
softer parts of the shell, do not become pressed into flat 
plates, as in the densest parts, but which are still so 
crowded together, as to be prevented from retaining the 
spherical figure. Besides these forms of carbonate of lime 
in the shell of the oyster, there is yet another form which 
is neither globular nor distinctly crystalline. It is the 
amorphous carbonate before alluded to, as occupying some 
of the interlaminar spaces in the place of the alkaline 
fluid which is contained in the other spaces. This amorphous 
carbonate can only result from the mixture of successive 
portions of water, contaiming in solution the compounds 
of lime, with the solution of alkaline carbonate formed on 
the surface of the animal. These two fluids finding 
access to these cavities, and the latter bemg probably 
deficient in animal matter, a decomposition takes place, 
and a carbonate is formed, not of the globular but of the 
amorphous character. Occasionally, in the thin plates of 
oyster-shell, branching dark lines are seen, resembling, in 
their mode of ramification, Conferve. These, in the 
perfectly calcified shell, are so blended with the contiguous 
calcareous plates, as not in such cases to admit of being 
