PTEROPODA. 4 
CLASS II. PTEROPODA. 
(Fin-footed Mollusks.) 
THIS is a very small Class, comprising a few 
species of curious structure. They are all of di- 
minutive size, and all swim in the open ocean, 
rarely approaching the shore, except when washed 
thither by accident. They are all characterised by 
having a membranous expansion, resembling a 
large fin, on each side of the head. By means of 
these organs, the little Pteropod rows itself about 
in the open sea perpetually; beg unfurnished 
with any means of crawling, or of affixing itself to 
any solid body. Some of these animals, as the ge- 
nera Hyalea (a) and Cleodora (b), for example, have 
the body enclosed in a shell of elegant form, and of 
GLASS SHELLS. 
a texture resembling the thinnest glass, for delicacy 
and transparency. The Cleodora pyramidata, one of 
the species of the latter genus, is of extreme delicacy 
and beauty. The shell is glassy and colourless ; 
