INTRODUCTION. 



Linneus in his system of nature, united all 

 Animals destitute of a spinal column and articu- 

 lated limbs, into a single class, of which 

 Testacea, or creatures having' an external shell 

 constituted an important division ; but since his 

 time the researches of Naturalists have proved, 

 that such an arrangement is inconsistent with 

 natural order. 



Of the Mollusca or soft bodied animals, com- 

 prizing within it this entire group, nearly all fiave 

 a developement of the skiu which covers their 

 body, and which bears more or less resemblance 

 to a mantle. The naked Mollusca are those in 

 which the mantle is simply membranous or 

 fleshy ; most frequently however, one or several 

 layers of a substance more or less hard, is formed 

 in its thickness, and increases in solidity as well 

 as in extent, because the newer deposits always 

 overlap the old ones. When this substance re- 

 mains covered by the thickness of the mantle, it 

 is still the custom to call the animals naked 

 Mollusca. More generally however, it becomes 

 so much developed, that the animal finds shelter 

 beneath it ; and the covering is then termed a 

 shell: the creature being denominated Testaceous, 

 or in common language, a shell fish. It is rare 

 however, to find a shell in which some filmy 

 portion of the soft animal substance with which 



