OROANIG STB UCTUBE OF SHELLS. 313 



A species of shell, the Cerithium telescopium, is so abundant 

 near Calcutta as to be used for burning into lime. Great 

 heaps of it are first exposed to the sun, to kill the animals, 

 and then burnt. In some places they are so plentiful as to 

 be used in road making. Mobile, Ala., is built on a shell- 

 bank. 



It was formerly believed that shells were not only devoid 

 of vessels, but completely without organs, being composed 

 of the transpiration of particles, chiefly carbonate of lime, 

 cemented together by a kind of animal glue. It is now 

 known that shells possess a more or less distinct organic 

 structure, which in some cases resembles that of the external 

 skin of the higher animals, while in others it approaches to 

 that of the true skin. 



In the limited space to which our remarks on the subject 

 of this chapter is necessarily confined, we cannot give more 

 than a brief outline of this exceedingly interesting depart- 

 ment of science. We may briefly observe that what are 

 •called the Testacea (a shell), comprise animals surrounded 

 with a shelly covering, and may be generally described as of 

 three kinds : those that possess a single shell, of whatever 

 form or character, and hence called univalves; those which 

 have two shells, the bivalves, or Conchce; and others having 

 more than two shells, or multivalves. Of these, the uni- 

 valves are the most numerous and exhibit the greatest 

 variety of forms, being for the 'most part regularly or irreg- 

 ularly spiral. Among the most common may be mentioned 

 the ZTeZiOJ, or snail genus ; the Paletta^ or limpet; and the 

 Turbo, or wreath genus, of which the periwinkle is a species. 



The shell of the Clam, or Bear's Paw, is described as, 

 perhaps, the most ornamental of bivalves, in regard to form, 

 texture and color. It comes from the South Seas, and is 

 much used for decorative purposes. 



Among the most curious shells is the Murex, or Purpie- 

 shell, so highly valued by the ancients for the exquisite dye 



