BY MOLECULAR COALESCENCE. 79 
I shall consider all these parts separately, in the order 
I have named them. And first, the tegumentary part of 
the shell, which is made up of alternate layers of hard and 
soft tissue, very closely connected together, especially in 
the more superficial regions of the shell. The intimacy 
of the connexion of these layers differs in different crus- 
taceans, and in different parts of the same shell. In some 
they are so blended together as not to admit of the 
slightest separation, excepting on the surface next to the 
animal. The first or deepest layer consists entirely of 
branched pigment-corpuscles. This‘is absent in the 
shells of many crustaceans, and therefore is not an essential 
part of shell-tissue. The next layer is a thin and almost 
transparent membranous layer, of a glassy appearance, 
and of rather a dense structure; connected on its deep 
surface with the pigment-layer, and by its superficial one 
with a layer of similar structure, sometimes several such 
layers may be found connected together. It is between 
these layers that the calcareous matter is deposited ; 
beginning on that surface of the deepest of these layers 
which is furthest from the branched pigment-corpuscles. 
The carbonate of lime here deposited is in extremely 
small particles, presenting when out of focus the appear- 
ance of minute points or perforations, dispersed more or 
less thickly over the membrane, though sometimes 
collected in groups, and at others arranged in lines. This 
deposit I have never found on the deep surface of the 
first layer, and sometimes two or three membranous layers 
intervene between the earthy deposit and the pigment- 
membrane. The particles of carbonate of lime as thus 
deposited, are sometimes too minute sensibly to polarize 
light; and in this case their composition can only be 
determined by the action of weak hydrochloric acid upon 
