CHAPTER XVIII. 



OF THE LAMELLIBRANCHIATE CONCHIFERA. 



r 930. THIS group is nearly synonymous with that of Bivalves 

 in the Linns3an arrangement, since all the animals which con- 

 struct bivalve shells belong to it ; but it also contains a few 

 species whose shells are Incottwalve ; and some others, in which 

 there appears at first sight an entire departure from the usual 

 form. The Mollusks belonging to this Class are, in common 

 with the Tunicata, destitute of a head ; that is, the mouth is 

 not situated upon a prominent part of the body, nor assisted in 

 its choice of food by organs of special sensation in its neighbour- 

 hood ; but the entrance to the stomach is buried between the 

 folds of the mantle. 



931. The part of the structure of these animals which is best 

 known, is the shell. This is composed of particles of carbonate of 

 lime, exuded from the surface of the mantle, and contained in the 

 cavities of cells, or between layers of membrane ( 870). If one of 

 the valves of an Oyster be examined, it will be seen to consist 

 of a number of layers, of which the external one is the smallest, 

 each inner one projecting beyond the one which covers it. This 

 is the case with other Bivalves ; but it is more evident in such 

 shells as that of the Oyster, in which the layers adhere loosely 

 together, than in others in which they are more compact. The 

 shelly matter is thrown out at intervals from the surface of the 

 mantle ; and as the animal enlarges at each interval, the new 

 layer extends beyond the old one. In this manner a constant 

 relation is preserved between the size of the animal and that of 

 its shell ; and the addition of the newly-formed portions, not to 

 the edge only, but to the interior of the whole previous shell, 

 strengthens the latter in proportion to its increase in size. 



932. The valves are connected together in various ways. 



