SYSTEMATIC DISTRIBUTION. 3 



vered with two separate shelly valves : they 

 live attached to other marine bodies. 



IV. PTEROPODA or Pteropods, having a promi- 

 nent head, with one or two pairs of fins on 

 the sides of the neck, by means of which 

 they swim about on the ocean ; body is often 

 covered with a thin glassy conoidal shell. 



Y. CEPHALOPODA or Cephalopods, which have a 

 large distinct head, furnished with eight or 

 ten arms, by means of which they walk head 

 downwards. 



The terrestrial and fluviatile Mollusca, of which 

 alone we have to treat in this little work, are con- 

 fined to the two first of these classes. 



The SHELL, which is peculiar to this division of 

 the animal kingdom, may be seen covering the young 

 animal in the egg, before it has gained all its organs, 

 as was observed by Swammerdam, and verified by 

 the more extended observations of PfehTer, Turpin, 

 and others. This may easily be seen in the egg of the 

 LimncBi, Physa, Ancyli, and Bitkinicp, which have a 

 transparent coat. 



The shells of the newly-hatched animals have 

 been frequently considered as distinct species ; and 

 some very thin shells of land Mollusca, such as 

 VitrincB, have been taken for the young of other 

 well-known species, as H. hortensis. These young 

 shells are easily known by their always being of a 

 pale horn colour ; the whorls are generally rather 

 irregular, and enlarge very rapidly; and the apex of 

 the whorl which was first formed is generally large 



B 2 



