122 C O N C H O L O G ¥• 



All fhell-like opercula are of a calcareous 

 nature, and diffolve in acids. It is there- 

 fore, that when put in vinegar or other 

 acids, they move briikly to and fro for fome 

 time,by the ebullition; from which particu- 

 lar, among the common people fond of cu- 

 riofities, they have obtained the name of 

 Creeping Stones. The horny and leathery 

 opercula fpurn acids. They have a kind 

 of greafinefs or unftuofity, which, when 

 they are burnt, exhales a ftrong fniel), 

 fometimcs agreeable, but moft generally 

 very foetid. The Blatta Byzantia, Con- 

 chylium, or Unguis Aromaticus of the 

 antients, and greatly valued, till of late, 

 in the materia medica, was of this latter 

 kind. It was called Unguis, becaufe ima- 

 gined to refemble the talons of a bird of 

 prey. Diofcorides propofes two kinds; one 

 from the Red Sea, white and greafy, which 

 was the moft efteemed : the other black 

 and not fo large, which came from Ba- 

 bylon. Of later times they have ufed 

 indifferently the fmall round opercula of 



Purpur?«9 



