BY MOLECULAR COALESCENCE. 83 
perfect shell by the globular carbonate of lime by which 
they are more or less surrounded. In the claw of the 
lobster the globular and laminated forms of the carbonate 
of lime are much less regularly disposed than in the crab 
but in good specimens of both the deepest layer contains 
very good examples of globules of carbonate in their 
several states of coalescence. I have in my possession 
specimens of vertical sections of lobster-shell, showing 
globular particles of various kinds, resembling, in all 
respects, the artificial globules, and not distinguishable 
from them even by the aid of the microscope and polarized 
light. Thin vertical and horizontal sections of the claw 
of a craw-fish, about three or four inches in length, 
or of a very small crab, examined in glycerine, are well 
adapted for displaying the process of development of shell. 
External to the membrane lining the cavity in the claw of 
such a crustacean, the particles of carbonate of lime coa- 
lesced into globules of different sizes may be seen to be 
collected together in one layer. The globules composing 
this layer being joined together only by their contiguous 
sides, present, by their unattached portions, undulating 
borders, partaking of the general form of the claw; one 
border being parallel with its cavity, from which it is 
separated by the deepest layer of membrane, and the other 
being parallel with the surface of the claw. I may observe 
that this is the general form and disposition of the coa- 
lescing globules, but it may be variously encroached upon. 
in different parts of the substance of the shell by small 
‘circular groups of coalescing globules, which have no 
connexion with the cavity of the claw, the convexity of 
these groups being turned towards this cavity, so that the 
radiating fibres of which the individual globules are made 
up being all directed towards its concavity, that is, from 
