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.t 

 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. SPH^ERIID^E. 

 II. UNIONHLE. 

 III. DREISSENID^:. 



Having two valves. f With leaf-like gills. 



R 



