488 FRESH-WATER AND SEA SHELLS. 



The fluviatile and lacustrine are usually combined under 

 the designation of. fresh-water shells, for there are no charac- 

 ters in the shell to tell us whether the river or the lake has 

 been its birthplace. Fresh-water shells are bivalve or uni- 

 valve and spiral, a very few only being simple univalves. 

 They are usually light of texture, and very generally of a 

 greenish or brown horn-colour; and their external surface 

 is plain, unornamented with varied or lively tints, and desti- 

 tute of varices, ribs, and spinous panoply. Very often the 

 apices of fresh-water shells are corroded as if they had been 

 worm-eaten, when they are said to be carious. Some authors 

 have asserted that the corrosions are produced by other Mol- 

 lusca, or by worms, attempting to destroy the inmate ; but 

 it seems due to the operation of some chemical agent, ac- 

 cording to Deshayes. The real agent is unknown; and 

 this is a subject that may be recommended to your early 

 examination. In entering upon it, one must bear in mind 

 the interesting researches of Mr. Albany Hancock, demon- 

 strative of the power the siliceous sponges have of making 

 somewhat similar excavations in marine shells. 



We approach the maritime shells through a tribe which 

 love the brackish waters, created at the mouths of rivers by 

 the mixture of their water with that of the sea ; and these 

 shells have the mixed and uncertain character of their posi- 

 tion. Look to the dark-hued Melaniae and to the Ceri- 

 thia. Littoral shells have the decided features which a sea- 

 origin gives to shells, and which it is in general easy to recog- 

 nise ; but not always, for there are some on whose habitat 

 conchologists have not yet ventured to decide. The Cyclo- 

 stoma flavum is a terrestrial snail whose shell " has all the 

 aspect of a marine" one, bearing " a close resemblance to 

 Littorina, especially in its operculum ;" * and the Natica 

 helicoides was pronounced to be a Paludina, until it was 

 discovered to be the inhabitant of sea-banks at a depth of 

 full thirty fathoms. The principal feature of a sea-shell is 

 the density and whiteness of its structure, and the varied 

 and vivid colouring of its outer surface. These colours 

 are modified a good deal by latitude, light, and by the depth 

 of water in which sea-shells are bred and pass their lives. 

 How varied, vivid, and beautiful are the testaceous Mollusca 

 of tropical seas and of tropical climes ! How sober and 

 subdued are those which inhabit our northern shores ! The 

 terrestrial Helices, being most exposed to the operation of 

 light, vary most in their colours ; while those shells which 



* Broderip in Proc. Zool. Soc. ii. 59. 



