564 



That It is difficult to difcover fpecific charafters of 

 diftinftion in Tome of the produftions of nature, all na- 

 turalifts will allow; and when no permanent diftinguifl".- 

 ing marks appear, wc can only refer to habits: in fol- 

 lowing this courfe there fe^ms nothing to divide thefe 

 two {hells from each other, nor from that which we 

 have defcribed as the ovalis : all the gradations that join 

 them are commonly found in the fame waters. That the 

 foil and nature of the water have confiderable effe£l 

 upon (hells, there can be no doubt, particularly in 

 Ilrength, and colour; for inftance, thefe fubjefts, which 

 are figured in the Britijh Shells, with elegant yellow ra- 

 dii, are rarely found in the Avon with fuch markings; 

 and inftead of the ground colour being green, are ufual- 

 ly of a dark olive-brown, and frequently dufky. 



The animals of thefe frefli-water Mycs, are certainly 

 not Afcidia;, but are fimilar to thofe inhabiting the river 

 Mytili. 



SuBORBicuLARis ^j^j^ j|^^|,^ ^^j^j^j^ -^ ^^ ^^^jj^^. j^ ^ ^^ ^,^^ Cardiuvi 



P' '^ ' corncum, is effentially different in the animal inhabitant, 



which appears to be a Tethys, of a very pnle colour, with 

 one long tube, not internally divided, but furntflied with 

 only one opening: and about the middle of the (hell in 

 front, is a retraftile, flender foot, half as long as the 

 fyplion, capable of adhering to the fmoothefl fur face, and 

 will afcend the fides of a glafs filled v/ith fea- water. The 

 animal of C. corneum is fimilar to that of C. lacuftre. 



SOLEN. 



