NEOLAMARCKISM 419 
‘“ The tendency to equalize the form of growth in 
a horizontal plane, or the geomalic tendency of Pro- 
fessor Hyatt,* is seen markedly in pelecypods. In 
forms which crawl on the free borders of the valves, 
the right and left growth in relation to the perpen- 
dicular is obvious, and agrees with the right and left 
sides of the animal. In Pecten the animal at rest 
lies on the right valve, and swims or flies with the 
right valve lowermost. Here equalization to the 
right and left of the perpendicular line passing 
through the centre of gravity is very marked (espe- 
cially in the Vola division of the group); but the in- 
duced 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 per- 
pendicular. In this case the geomalic growth corre- 
sponds 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 con- 
cave, the free valve flat; but they are not only un- 
equal, 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 
* «Transformations of Planorbis at Steinheim, with Remarks on 
the Effects of Gravity upon the Forms of Shells and Animals,” Pro- 
ceedings A. A. A. 5., xxix., 1880. 
