120 C O N C H O L O G Y. 



wrought with a fpiral work, or with con- 

 centric circles. 



The operculum, or Hd, is always fixed 

 on the upper part of the pedeftal of the 

 fifh. In fome at the outer end or extre- 

 mity, fo that it retires confiderably from 

 the Shell when the animal moves or walks. 

 In others it is placed at the inner extre- 

 mity or root. The operculum exactly 

 covers or clofes the Shell in thofe whofe 

 mouths are round, femicircular, or oval, as 

 the Nerits, PL III. fig. 14. Turbines, 

 PI. Ill, fig. 12. Purpuras, PL IV. fig. 4, 

 and ^, &c. ; but in thofe Shells that have 

 very lengthened or narrow mouths, as the 

 Volutes, PL V. fig. 8 and 9, it is not 

 eafy to conceive what ufe the opercuLi 

 ^re gf ; for they feem not to fhut or co- 

 ver much above the fifth part of the 

 fnouth *^ 



The 



* Surely all the Operciilae fervc as covers, and en- 

 tirely fliut up the fiili ; therefore, though they do not 



fcetu 



