Professor Forbes, in a communication to the Royal Society, states that
not only the colour of the shells of existing mollusks ceases to be
strongly marked at considerable depths, but also that well-defined
patterns are, with very few and slight exceptions, presented only by
testacea inhabiting the littoral, circumlittoral, and median zones.
In the Mediterranean, only one in eighteen of the shells taken from
below 100 fathoms exhibit any markings of colour, and even the few
that do so are questionable inhabitants of those depths. Between 30
and 35 fathoms, the proportion of marked to plain shells is rather
less than one in three; and between the margin and two fathoms the
striped or mottled species exceed one-half of the total number. In our
own seas, Professor Forbes observes that testacea taken from below 100
fathoms, even when they are individuals of species vividly striped or
banded in shallower zones, are quite white or colourless. At between
60 and 80 fathoms, striping and banding are rarely presented by our
shells, especially in the northern provinces; from 50 fathoms, shallow
bands, colours, and patterns, are well marked. _The relation of these
arrangements of colour to the degree of light penetrating the different
zones of depth_ is a subject well worthy of minute inquiry.