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. MoLLUSCA — 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, 

 whicii 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, as hyaleea. 

 " IV. Brachiopoda — arm-footed, as terebratula. 

 " V. CoNCHiFERA — shell-bcarers, 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 laminee. 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 



B 



