ON MOLLUSCA. 165 
tems or arrangements of coloration, sometimes uniform, some- 
times pitted, or spotted, sometimes striped longitudinally or 
transversely. 
Finally, a last part which enters into the composition of 
shells is the epidermis, which covers the colouring pigment, 
and which is sometimes named epzphlosis. It is evidently 
in the epidermis of the skin that the shell is deposited. This 
epidermis is formed of a mucous or corneous matter dried up ; 
sometimes producing a stratum more or less thick and smooth 
at the surface of the shell, and sometimes rising in lamine, or 
filiform productions, flatted or conical, and sometimes elon- 
gated, so as to resemble kinds of hairs. In the bivalves this 
part is of the same nature as the ligament, and it sometimes 
envelopes the valves altogether, as in solen. It is this part 
which first begins to be formed in the growth of a shell, uni- 
valve or bivalve, and whether it is to remain with an epidermis 
or not. 
After what we have said concerning the structure of the 
shell of the mollusca, it is certain that it is chemically com- 
posed of two substances; 1. of an animal mucous matter, more 
or less abundant, according to the age of the molluscum, the 
part of the shell analyzed, and its structure ; 2. of a calcareous 
salt, but which varies in quantity according to the age of the 
conchyliferous mollusca. Although the analysis of shells 
given by chemists is very incomplete, inasmuch as it is exer- 
cised on all their parts at once, without distinction of age, we 
can nevertheless recognize that the differences in the results 
are pretty much in relation with the differences of structure. 
The species which contain in general the largest portion of 
animal matter appear to be those which possess the fibrous 
and nacreous structure. According to Mr. Hatchett they are 
formed of subcarbonate of lime and coagulated albumen. ‘The 
mother-of-pear! itself, in 100 parts is composed of 66 of the 
first and 34 of the second. 
