FOSSIL INVERTEBRATED ANIMALS. 487 



The form of individuals of the same species is often so varied, 

 that it is very difficult to distinguish the different species^ espe- 

 cially when they have some relation between each other, which 

 often happens. 



Some species of oysters, not foliated, and which have the 

 faculty of attaching themselves on other bodies, by a consi- 

 derable portion of their lower valve, not only modify this valve, 

 which, at its external part, represents the hollow mould of the 

 body in which it has been applied, but copy this body exactly, 

 in relief, on the upper part of the upper valve. Many may be 

 seen which have adhered on pectines, turritellse, polyparia, and 

 other bodies, which are figured on the upper valves. This is a 

 property which the oysters have in common with some species 

 of gryphsese, of anomise, and balanse. Some of these mollusca, 

 in the living state, possess the faculty of giving to their shells 

 the colour of those in which they have adhered. 



Some fossil oysters exhibit, in a very obvious manner, the 

 kind of mechanism by which they destroy the hinder part of 

 their adductor muscle, while they advance that which is ante- 

 rior. There are certain species in which this muscle has been 

 displaced several inches, to be removed from the heel against 

 which it was situated at the birth of the animal, and carried 

 forward, in proportion as the latter increases in growth. In 

 forming each of the calcareous layers which have given thick- 

 ness and extent to the shell, the molluscum displaces itself to 

 get forward; thus abandoning the heel, where it sometimes 

 leaves vacant septse. The adductor muscle being attached to 

 the valves by each of its ends, it cannot make this displace- 

 ment except in consequence of the augmentation of the anterior 

 part of this muscle, and its attachment to the new layer; while, 

 at the same time, that which is opposite to it is destroyed, in 

 almost an equal proportion, by a trenchant calcareous plate, 

 which follows it in its displacement, and frequently does not 

 touch the bottom of the shell. Certain species have the faculty 



