INTRODUCTION. 
Uinneus in his system of nature, united all 
Animals destitute of a spinal column ancl 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 have 
a developement of the skin which covers then- 
body, and which bears more or less resemblance 
to a mantle. The naked Mollusca are those in 
'vhich the mantle is simply membranous or 
heshy ; most frequently however, one or several 
layers of a substance more or less hard, is formed 
its thickness, and increases in solidity as well 
as in extent, because the newer deposits always 
overlap the old ones. When this substance re- 
gains covered by the thickness of the mantle, it 
is still the custom to call the animals naked 
Mollusca. More generally however, it becomes 
s ° 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 
