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. S., xxix., 1880. 



