222 BERMUDA. 



are exhibited by circular grooves ia the shell. The 

 beautiful little shell belonging to this species is 

 occasionally picked up along our shores after heavy 

 storms. 



To the second class of MoUusca belong the Ptero- 

 poda. This is a very small class, comprising a few 

 species of curious structure. They swim, hke the 

 Cephalopoda, in the sea, but cannot fix themselves 

 there, nor creep, from want of feet. They are all 

 characterized by having a membranous expansion, 

 resembling a large fin, on each side of the head. 

 Some genera only are furnished with a thin, cartila- 

 ginous, or horny shell. 



The only genus known here is the LimacincB, 

 which is represented by a single species — Limacina 

 Jielicialis. Its body is terminated with a spiral tail, 

 and is lodged in a very thin shell, of one whorl and 

 a half. The shell serves the purpose of a boat ; and 

 when the creature wishes to swim on the surface, 

 it uses its fins as oars. 



To the third class of MoUusca belong the Gastero- 

 poda. Cuvier has given the name of Gasteropoda 

 to all the animals of this class which have a foot or 

 muscular disk proper for crawling, whether this foot 

 extends the whole length of the lower surface of the 

 body, or adheres only to the base of the neck. The 



