164 SUPPLEMENT 
Light unquestionably has a very powerful influence on the 
coloration of shells, since those which are altogether interior, 
or deposited in some large lodge of the dermis, are always 
white, in the same manner as those of animals which live 
constantly in holes from which they never come forth. But 
another proof of this fact is, that in certain bivalve shells, 
which are always fixed somewhere, more or less horizontally, 
the fixed valve is constantly white, while the upper one is 
often coloured in a very lively manner. The spondyli, and a 
considerable number of pectines present examples of this. 
We must then admit here that one lobe of the mantle, not 
receiving the exciting action of the light, does not produce 
any coloured pigmentum, while the reverse is the case with 
the other; or, to express ourselves with more precision, the 
pigmentum is coloured by this action alone ; so that if one of 
those shells should be designedly reversed there would be a 
reverse in the coloration of the valves, as actually does take 
place with the sides of certain pleuronectes. 
In general the coloration of shells is so much the more 
lively as the animals from which they proceed are more ex- 
posed to the action of the light. The helices, terrestrial 
animals, are,in fact, those whose shell varies the most in 
colour; the tubicole, among the bivalves, have, on the con- 
trary, their shell constantly white. Olivi, who has made 
some researches on this subject, has equally remarked, that 
the shells which are enveloped by sponges or alcyones, or 
which live in sand, or even in places which are constantly 
shaded, are much paler than those which are constantly un- 
covered in places greatly exposed. The same shell too is 
often more coloured in its uncovered parts than in those which 
are concealed. 
We find almost all kinds of colour on the external surface 
of shells, most commonly however brown and fawn-colour, 
but green less frequently. There are a great number of sys- 
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