THE 



LAND AND FRESHWATER SHELLS 



OF 



THE BRITISH ISLES. 



AQUATIC. 



CLASS I.— CONCHIFERA, OR BIVALVES.* 



Body oval, sides usually compressed ; there is no distinct head ; 

 the mouth is situated within the folds of the mantle ; mantle 

 with two lobes ; in some species it is open, and its edges are then 

 usually fringed, in others it is furnished with tubes, or siphons^ 

 in which case the latter are frequently fringed with thread-like 

 filaments or feelers ; foot tongue-shaped, often capable of great 

 extension ; in some species it is provided with a tuft of threads, 

 called the byssus^ by which the animal attaches itself to stones 

 or other bodies ; both sexes are united in the same individual, 

 which has the power of fertilizing itself. The whole or greater 

 part of the body is enclosed in a shell composed of two pieces or 

 valves^ which are united by a ligament or hinge along their 

 dorsal margin. Respiratory organs consisting of gills. 



Order LAMELLIBRANCHIATA.f 



Gills four, leaf-shaped, two on each side of the body. 



All the British freshwater bivalves belong to this 

 order, and are divided into three families, viz. : — 



I. SpHzERIID/E. 



II. UnionidtE. 

 III. Dreissenid^. 



Having two valves. f With leaf-like gills. 



P. 



