9 



order of the fourth class of Vermes or Worms. In the Linnsean system of 

 Zoology, Shells are divided into 



Midtivalves, Bivalves, and Univalves. 



AMultivalve shell may be exemplified by any species of Lepas or Barna- 

 cle, in which the shelly covering of the animal is formed of several pieces or 

 divisions. The Bivalve, by the muscle in which, as every one knows, the 

 shell is composed of two pieces or valves ; and lastly, the Univalve shell by 

 the common snail, for the shell is simple and undivided. The shell animals 

 are produced from eggs, which in some animals are gelatinous or gluey, and 

 in others covered with a hard or calcareous shell, and the young animal 

 emerges from the egg with its shell upon the back. The most familiar and 

 convincing proof of this may be obtained by observing the evolution or hatch- 

 ing of the eggs of the common garden snail, as well as several of the water 

 snails, which deposit eggs so transparent, that the motions of the young, with 

 the shell on its back, may be very distinctly seen several days before the 

 period of hatching. 



All the shell animals are of such a constitution as perpetually to secrete or 

 exude from their bodies a viscid moisture ; and it is with this, managed ac- 

 cording to the exigencies of the animal, that the shell is through life increased 

 in dimensions, and repaired when accidentally broken in any particular part. 

 The growth of shells proceeds from the edges of the mouth, or opening ; and 

 thus the spires or turns of the Univalve shells are gradually increased in 

 number and size, till the animal has arrived at its full growth. 



Linnaeus has divided shells into thirty-six genera; and the total amount of 

 species hitherto described is 2445, exclusive of varieties, of which 550 are to 

 be found in and around the British Isles. There are many species, no doubt, 

 that have not yet made their way into the catalogues of Conchological writers; 

 and we have reason to suppose that hundreds or thousands may be still un- 

 known. 



