236 MOLLUSCA. GASTEROPODA 



as the operculum, attached to the dorsal part of the posterior 

 end of the foot ; this is so arranged that when the animal 

 withdraws into its shell the operculum more or less com- 

 pletely closes the aperture. It has been considered by some 

 as a second valve, but more probably represents the byssus 

 of the lamellibranch. The operculum is seldom preserved 

 fossil ; its form varies considerably in different genera, in 

 some {Turbo) it is of very large size with the inner surface 

 flattened and the outer convex; it may have a spiral 

 structure, and is then sometimes formed of a large number 

 of whorls (midtispiral) as in Trochus, or of a few whorls 

 (paucispiral) as in Littorina. When not spiral it may be 

 concentric, if growth takes place equally all round ; it is 

 then marked with concentric lines, the nucleus being 

 nearly central, as in Viviparus ; or it may be unguiculate 

 or claw-shaped when the nucleus is at the apex as in 

 Fusus. 



The form of the shell in the spiral gasteropods varies 

 considerably, depending on the arrangement of the whorls 

 in one plane or in a helicoid spiral, on the spiral angle, on 

 the number and shape of the whorls, on the size of the last 

 whorl and whether it conceals the earlier whorls or not. 

 The chief types are the following : — 



1. Discoidal ; the whorls all in one plane, as in 

 Planorbis. 



2. Conical or trochiform ; conical with a flat base, as 

 in Trochus. 



3. Turbinate; conical with a convex base, as in 

 Turbo. 



4. Turreted or elongated ; as in Turritella. 



5. Fusiform; tapering to each end, as in Fusus. 



6. Cylindrical ; as in Pupa. 



