BRITISH LAND AND FRESH WATER SHELLS. 



Naturalists have divided all living, sentient beings, 

 into four sub-kingdoms, according to the different types 

 upon which they are constructed, viz., 

 SUB-KINGDOM. I. VERTEBRATA backboned animals. 

 II. MOLLUSC A soft or pulpy animals. 

 " III. ARTICULATA jointed animals. 



IV. RADIATA rayed, or branched 



animals. 



The animals under consideration in the present vo- 

 lume, belong to Mollusca, the second sub-kingdom, 

 which is divided into the following six classes : 

 CLASS I. CEPHALOPODA head-footed, as cuttle fish. 

 " II. GASTEROPODA stomach-footed, as common 

 snail. 



III. PTEROPODA wing, or fin-footed, ashyalsea. 



IV. BRACHIOPODA arm-footed, as terebratula. 

 " V. CONCHIFERA shell-bearers, as all bivalves. 



VI. TUNICATA membrane-covered, porophora. 



The two classes, Gasteropoda and Conchifera, em- 

 brace all the land and fluviatile species. 



Commencing at the foot of the scale, the bivalve shells 

 will come first under notice. 



CLASS V. CONCHIFERA. 

 SECTION I. ACEPHALA LAMELLIBRANCHIATA. 



So called from the animal having no head, and its 

 breathing apparatus being expanded into laminae. The 

 mouth is situated between the four leaflets of the bran- 

 chia, with a central foot more or less distinct, and com- 

 pressed, covered by two pair of leaf-like gills, and large 



