. Here equalization to the right
and left of the perpendicular line passing through the centre of
gravity is very marked (especially in the Vola division of the
group); but the induced right and left aspect corresponds to the
dorsal and ventral sides of the animal, not the right and left
sides, as in the former case. Lima, a near ally of Pecten, swims
with the edges of the valves perpendicular. In this case the
geomalic growth corresponds to the right and left sides of the
animal.

"The oyster has a deep or spoon-shaped attached valve, and a flat or
flatter free valve. This form, or a modification of it, we find to
be characteristic of all pelecypods which are attached to a foreign
object of support by the cementation of one valve. All are highly
modified, and are strikingly different from the normal form seen in
locomotive types of the group. The oyster may be taken as the type
of the form adopted by attached pelecypods. The two valves are
unequal, the attached valve being concave, the free valve flat; but
they are not only unequal, they are often very dissimilar--as
different as if they belonged to a distinct type in what would be
considered typical forms. This is remarkable as a case of acquired
and inherited characteristics finding very different expression in
the two valves of a group belonging to a class typically
equivalvular. The attached valve is the most highly modified, and
the free is least modified, retaining more fully ancestral
characters. Therefore, it is to the free young before fixation takes
place and to the free, least-modified valve that we must turn in
tracing genetic relations of attached groups. Another characteristic
of attached pelecypods is camerated structure, which is most
frequent and extensive in the thick attached valve. The form as
above described is characteristic of the Ostreidae, Hinnites,
Spondylus, and Plicatula, Dimya, Pernostrea, Aetheria, and Mulleria;
and Chama and its near allies. These various genera, though
ostreiform in the adult, are equivalvular and of totally different
form in the free young. The several types cited are from widely
separated families of pelecypods, yet all, under the same given
conditions, adopt a closely similar form, which is strong proof that
common forces acting on all alike have induced the resulting form.
What the forces are that have induced this form it is not easy to
se