NATURAL HISTORY. 



Order GASTEROPODA. <Gr. Taarqp, the belly; TTOVC, a foot.) 



Sub-order.PuLMOBRANCHiATA. (Pulmo the lungs; Iranchice, gills.) 

 Family. ... Liraacidae. (Gr. Ae//af, a Slug.) 

 LIMAX. 



Ater (Lat. black), the Black Slug. 



Shells are secreted from a part of the inhabitant called the 

 " mantle," and of course, as the .shell is always added round 

 the rim, as may be seen by taking a small snail in the spring 1 , 

 it naturally follows, that as the animal becomes larger, so the 

 mantle becomes larger, and secretes a larger ring of shell. 



Many shells, as that of the oyster, are deposited in layers, a 

 fine membrane interposing between each layer : they are there- 

 fore called membranous shells. Most membranous shells are 

 lined with a brilliant enamelled substance, called "nacre:" 

 " mother of pearl" is the nacre of the pearl oyster. That of 

 the fresh-water mussel is a beautiful azure. 



The other structure of shells is called " porcellaneous," be- 

 cause they look like porcelain or china. The common cowrie 

 is a well-known instance of a porcellaneous shell. Some shells 

 are so transparent as to resemble glass, and are therefore called 

 " vitreous."* 



Shells are divided into Univalve, or one-valved shells, such 

 as the snail ; and Bivalve, or two-valved shells, such as the 



* Derived from Lat. vitrum, glass 



