SECTION V. 121 



The above is meant only for Sea Uni- 

 valves, whofe opercula are a part of the 

 animal, and is brought forth with it. The 

 operculated Land Univalves are very differ- 

 ent; they form a new lid or operculiini 

 every year, or oftner ; and that is only 

 at fuch times that the animals want to 

 fhelter themfelves from the injuries of 

 the w^eather. It is compofed of a vil- 

 cous flabber, from the body of the animal, 

 which condenfes into a kind of toughifh 

 coriaceous or leather-like fubftance, and is 

 pretty thick. This lid or cruft is never- 

 attached to the body of the animal, as in 

 the Sea Univalves, but merely covers the 

 mouth ; nor is it ever wrought with a fpi- 

 ral or with concentric circles, or indeed 

 any other regular work. 



(eem to fit the outer mouths or apertures of the 

 Shells, yet the fifh retires within the Shell, fo far as 

 U) make it fit, or clofe exadly to where he retires. 



All 



