238 



CHAPTER YII. 



Class III. PTEROPODA. 



" The sea ! tlie sea ! the moonlit sea ! 

 How calm its slumbering tide," 

 And, floating o'er the wide expanse, 

 Small boats in safety glide. 



The limited class Fteropoda (from two Greek words, sig- 

 nifying wing diXidi foot) contains a company of small twilight 

 deep-water swimmers, which obtain their powers of loco- 

 motion, as their name indicates, by aid of a pair of fins, 

 somewhat resembling wings, and which alone produce a 

 small, brittle, glass-like shell. 



Cuvier was the first who treated them wdth any degree of 

 scientific accuracy ; before his time their existence was 

 comparatively unknown, but since then the researches of 

 D'Orbigny and Souleyet have brought to light many curious 

 facts associated with their habits and geography. 



