90 FORMATION OF SHELLS OF ANIMALS, ETC., 
being as before described. It may be observed, further, 
that interstices of precisely the same character as those ex- 
isting between the particles of dentine are found also be- 
tween the analogous portions of enamel, which are also 
considered by some anatomists as distinct tubes. The 
part of the shell of this class of animals next requring 
notice is that which is situated between the muscles, giving 
attachment by each of their surfaces to muscular fibres, 
and therefore, both in situation and function, being ana- 
logous to bone. I allude to the parts called by anatomists 
“‘apodemata.” These parts are made up of layers of cal- 
cified membrane rather loosely connected together, having 
the appearance when dried of pieces of white paper stuck 
together. The membranous layers of these parts, when 
just beginning to calcify, exhibit all the stages of calcifi- 
cation very distinctly. In no part are the facts showing 
molecular coalescence better seen, from the minutest par- 
ticles up to perfect globules, with lamine and radiating lines, 
and, if the part be sufficiently thick, the coalescence of these 
globules to form layers made up of the horizontal undulating 
lines, called lines of growth, and the perpendicular sinuous 
lines, or the so-called tubules of physiologists can be well 
seen. These parts require no other preparation than 
that of drying the shell, and then splitting off the thinnest 
possible pieces, which may be examined either in glycerine 
or turpentine. It is scarcely necessary to state, that these 
parts, in all shells, are not equally favorable for exhibit- 
ing these appearances ; several, therefore, ought, if neces- 
sary, to be examined for that purpose. Of course the 
regular tubes, mentioned as existing in the tegumentary 
part of the shell, passing from its deep to its superficial 
surface, are absent in these parts; but, m other respects, 
their structure is similar. The last part of the shells of 
