104 FORMATION OF SHELLS OF ANIMALS, ETC., 
dence upon this point appears very clear and circum- 
stantial, it does not agree at all with the facts which 
occurred to me in examining the same parts, and there- 
fore I am obliged to differ from him. I have never seen 
in this membrane, when completely separated from the 
calcareous matter, any trace of this nacreous lustre, and 
consequently believe it to be due entirely to the carbonate 
of lime, and not to the membrane on which it is deposited. 
In my experiments I have always employed the polariscope, 
which furnishes the best means of deciding this pomt. More- 
over, the nacreous lustre of a piece of shell is not in the 
least impaired by boiling it for any Jength of time in hquor 
potasse ; and but little so by heating it to redness, which 
must have been the case if the lustre were produced by 
the delicate folds of what Dr. Carpenter has called the 
nacre-membrane. Now, from what has been stated con- 
cerning the chemical part of the process by which the 
carbonate of lime in a globular form is produced in the 
shells of crustaceans, but little remains to be added to 
what has been already noticed of the formation of this com- 
pound on the septa between the interlaminar cavities. A 
few words will finish this part of the subject. In animals 
of this class, all that surface which comes in contact with 
the shell is moistened with a solution of carbonate of soda 
and animal matter, which can be easily demonstrated, as 
before observed, by the proper tests. I may just notice, 
however, that a piece of reddened litmus paper, kept in 
contact with the surface of a mussel or oyster for a minute 
or two, never fails to show the presence of an alkali. I 
cannot say that this is the case only with the part con- 
nected with the shell, as I have not tried it on ail parts of 
these animals; but that does not at all affect this question. 
Besides this secretion of alkali, a membrane is formed on 
