108 FORMATION OF SHELLS OF ANIMALS, ETC., 
satisfactorily examined. However, in an imperfectly 
calcified layer, sufficiently recent, they can be so far 
detached, whilst in their growing condition, from the 
layers of shell, as to remove all doubt as to the fact of 
their being true Conferve. These growths are found on 
the surface of the calcifying membrane, which is exposed 
to the contact of the salt water, this being without doubt 
the medium by which they are brought, in the condition 
of sporules, into this position. The quantity of this 
confervoid growth differs very much in different shells. 
In shell-structures the earthy matter is considered by 
those who have written upon the subject, to be contained 
in the interior of cells. Dr. Carpenter, when treating of 
the prismatic cellular substance of shells, observes, that 
“The resemblance in their structure and position between 
the prismatic cellular substance of shells, and the prismatic 
epithelium covering the mucous membrane of higher 
animals, leaves no room for doubt of their analogy, and 
we may consider this form of shell-structure in the light 
of a calcified epithelium, the carbonate of lime being 
deposited in the cavities of the cells, and in general 
completely filling them. In most sections of Pinna a 
greater or less number of dark cells may be seen, which 
are usually disposed with some degree of regularity. 
These I have reason to believe to be cells mcompletely 
filled with carbonate of lime, the spaces remaining in 
which, being occupied by air only, present a dark ap- 
pearance under the microscope, according to the general 
principle well known to observers.” Now, according to 
this view of the structure of shells, which is I believe the 
one generally maintained, cells are formed first, and 
afterwards carbonate of lime is deposited in their cavities ; 
hence, in the process of development of  shell-tissue, 
