MOLLUSC A OR UNIVALVES. 157 



ticed by Lamarck, but called by English col- 

 lectors the gizzard. It is composed of three 

 bony pieces, united by a muscular membrane ; 

 the two side pieces are triangular, the other is 

 small. 



The B. Velum is a very delicate shell, of a 

 very light horn-colour, with a snow-white 

 band about the middle, bordered on each side 

 by one of a dark brown colour ; the apex and 

 base are white, both bordered by dark brown 

 bands. 



Family 7. — Aplysidce* 



The Mollusca constituting this small family 

 offer only two genera, and the shell with which 

 they are furnished, if it can be called a shell 

 at all, is a mere rudiment of one, forming 

 only a covering for the gills ; this at least is the 

 case with the first genus ; the other, in lieu of 

 this plate of defence for its breathing apparatus, 

 has a more defined shell, serving the same pur- 

 pose. 



31. Aplysia. — Shell, or operculum for gills, 



* Les Laplysiens, Lamarck. 



