INTRODUCTION. 11 



HABITS — Land, Fresh-water, or Marine Shells. 



Another distinction, leading to important results in classifi- 

 cation, is that which is derived from the nature of the element 

 breathed by the Mollusc. And although this consideration 

 belongs more especially to the study of the animal itself, yet 

 the habits of the animal materially influence the structure of 

 the shell. 



The Terrestrial or Land Molluscs live on land, breathe 

 air, and feed on plants and trees. — Those who find pleasure 

 in horticultural pursuits will at once call to mind a too familiar 

 example of these Molluscs in the common garden snail. The 

 Land-shells are all univalves, and constitute a family in the 

 Lamarckian system under the name " Colimacea" or snails, 

 corresponding with the Linnean genus Helix. — They are 

 generally light in structure and simple in form. 



The Aquatic, or Fresh-water Molluscs, such as the 

 Planorbis, commonly called the Fresh- water Snail; the Unio — 

 known by the name of Fresh- water Muscle, is found in 

 ponds, ditches and rivers. The epidermis of these is gene- 

 rally of a thick, close-grained character, and they are subject 

 to corrosion near the umbones. There are but few genera of 

 fresh-water shells besides the Uniones, among bivalves, and 

 the "Melaniana" among univalves. Concerning the former it 

 may be observed, that they are all pearly within, and the colour 

 of the thick horny coating embraces all the varieties of brownish 

 and yellowish green. 



The Marine, or sea-shells, belong to all the classes and 

 orders, and include by far the greater number of species. 

 They vary in the habits of the animal, and consequently in 

 the situations in which they are found. Some are found buried 

 in sand and marine mud, and are named " Arenicolce" or in- 

 habitants of sand ; others in holes of rocks and other hard sub- 



