GENERA OF SHELLS. 59 



SEMI-PHYLLIDIANA. 



PLEUROBRANCHUS., Shell internal, dorsal, 

 thin, flattened, often oblique, oval. 



P. plumula, Peronii. 



UMBRELLA. Shell external, orbicular, some- 

 what irregular, nearly flat, slightly convex 

 above, white, with a small apicial point in the 

 centre, edges sharp ; internal face a little con- 

 cave, with a callous, colored disc, pitted in the 

 centre, and surrounded by a smooth limbus. 



V. Indica, Mediterranea. 



CALYPTRICIANA. 



Shell always external, covering the animal. 



PARMOPHORUS. Shell oblong, subparallelo- 

 piped, somewhat convex above, blunt at ex- 

 tremities, notched anteriorly by a slight sinus, 

 a small apicial point above, towards the poste- 

 rior part, inclined backwards. 



P. australis, breviculus, granulatus. 



EMARGINULA. Shell conic, shield-like, vertex 

 inclined ; posterior margin split or emarginate* 



The shells of this genus are generally small ; some 

 of them in the form of a cone, inclined towards the 

 anterior margin, which is always the narrowest, and 

 opposite to that which has the fissure. In others, the 

 cone is very much flattened. 



E. fissura, rubra. 



FISSURELLA. Shell shield-like, or depressed 



