x891.) Mechanical Origin of Structure in Pelecypods. 19 
edly in Pelecypods. In forms which crawl on the free borders of 
the valves the right and left growth in relation to the perpendicu- 
lar is obvious, and agrees with the right and left sides of the ani- 
` mal. In Pecten the animal at rest lies on the right valve, and 
swims with the right valve lowermost. Here equalization to the 
right and left of the perpendicular line passing through the centre 
of gravity is noticeable (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, appar- 
ently 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 distinct species in 
what would be considered typical forms. This is remarkable as 
a case of inherited or acquired characteristics finding very dif- 
ferent 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 valve 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 struc- 
ture, which is most frequent and extensive in the thick attached 
valve. The form as above described is characteristic of the Os- 
treidz, Hinnites, Spondylus, and Plicatula, Dimya, Pernostrea, 
Etheria, and Mulleria, Chama and its near allies. These vari- 
ous genera, though ostreiform in the adult, are equivalvular and 
