118 MOLLUSCA. 



CLASS LAMELLIBRANCHIATA. 



These mollusks, called also Conchifera and Pelecypoda, are 

 the common bivalves. Tliej are all acephalous and aquatic, 

 breathing b}' leaf -gills, — whence the name,— usually a pair of gills 

 on each side of the body. The mantle, in this class, forms two 

 lobes either side of the animal, and secretes the shells upon their 

 outer surfaces. The shells are mostly symmetrical or equivalve, 

 the exceptions occurring in the stationary species. 



Each valve is subconical, inequilatei-al, and the two are articu- 

 lated, dorsally, by a ligament and teeth. The valves are closed 

 by one or tM'o powerful adductor muscles, which leave character- 

 istic muscular scars. Nearly all have a musculai- foot for loco- 

 motion, which in some groups contains a gland for secreting 

 tough fibres, the byssus, which serve as a fastening to the rocks. 

 The mantle lobes are sometimes prolonged posteriorly as tubes 

 (siphons), which enable the owner to breathe and feed while the 

 body is buried in the sand ; this being indicated by an inbending 

 of the pallial line. The Lamellibranchs do not readily group 

 themselves into natural orders ; the similarity of type is great, 

 but the points of difference are not constant. The usual division 

 is into the Asijphonida and Siphonida. The VeneridcB ai-e the 

 typical and most highly organized Lamellibranchs. 



The bivalves, though less numerous now specifically, are far 

 more abundant individually, than the gasteropods. They are all 

 marine, excepting a few widely dispei'sed fresh-water genera (10 

 out of 90), and are found on everj^ coast and in every climate, 

 and from low water to the depth of 200 fathoms. Tlie fossil 

 forms constitute a third part of fossil shells. The living species 

 number 4,000, and the fossil species about 7,800. The genera 

 are seven times more numerous in the newer Tertiary than in 

 the Silurian. 



