Pvn H,i I'UitlS

50 PECTINID^E.
frame of a fan. Scallops are especial favourites of shell-
collectors and amateurs, on account of their elegant
shape and their brilliant and varied hues. The curious
' '
organs called ocelli v or eyelets are supposed by some
physiologists to be rather highly organized, and even
superior to the so-called eyes of most Gasteropodous
Mollusca. More than one hundred of them have been
counted in a single individual of some species oiPecten.
For this reason Poli called the animal Argus. These
little eyes have a prismatic lustre, and gleam like precious
stones which are set round the inside of a casket
lined with mother-of-pearl. Their structure has been
lately and independently investigated by Grube, Krohn,
and Will. Very young shells of all the species are
destitute of ribs ; and they are nearly rhomboiclal, owing
to their breadth and the size of their ears being proportionally
greater at that stage of growth than afterwards
is the case. In consequence of the Scallops being generally
attached or sedentary, the upper valve is more
deeply and brightly
coloured than the lower one.
Although all the essential characters of the present
genus are uniform and do not vary much in the several
authors into no less
species, it has been divided by
than twenty-eight, most of which will be found enu-
merated in the useful Index of Herrmannsen. In nearly
all the British species the upper or left valve is the
larger, and is also distinguished from the other by its
brighter or deeper hue. In Pecten maximus, however,
and occasionally in P. septemradiatus, the lower or right
valve is the larger, and almost or quite colourless. The
intensity of colour is supposed to depend
of solar light, although it is not wanting
on the action
in animals
living in the abysses of the ocean, which the most attenuated
sunbeam has never directly penetrated.