154 MOLLUSCA. GASTEROPODA. 



In many genera there is attached to the posterior part 

 of the foot a calcareous or horny plate, the operculum ; 

 this is so arranged that when the animal withdraws into 

 its shell, the operculum closes more or less completely the 

 aperture. It has been considered by some as a second 

 valve, but it probably represents the byssus of the 

 lamellibranch, rather than this. The operculum is seldom 

 preserved fossil ; its form varies considerably in different 

 genera, in some (Turbo) it is of very large size ; it 

 may be spiral, and is then sometimes formed of a large 

 number of whorls (multispiral) as in Trochus, or of a few 

 whorls (paucispiral) as in Littorina. When not spiral it 

 may be concentric when growth is equal all round and it 

 is marked with concentric lines, the nucleus being nearly 

 central, as in Paludina ; 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 whether the whorls are in one 

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

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

 whorl and whether it embraces 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. 



7. Globular ; as in Natica. 



