476 



P. with a fub-conic fliell. fubjeft to much variety in 

 its degree of elevation, in the prominence and number 

 of ridges, and in the indentations at the margin ; fo much 

 fo as to have occafioned it to be muhiplied into two or 

 three different fpecies. The full grown fliells are ufually 

 of a conic fhape, and fomewhat oval ; the vertex approxi- 

 mating rather to the fmaller end, but frequently nearly 

 central, marked with numerous fine flria; from the apex 

 to the margin; which laft is fometimes indented a little, 

 fometimes quite plain : colour brown, or corneous. In- 

 fide fmooth, glofTy, and of a yellowifh horn-colour, 

 whitifh at the bottom. 



Others are more depreffed ; vertex placed more to 

 one end, and the whole fliell furniihed with fhong ele- 

 vated ribs, with intermediate flriec; the margin (Irortgly 

 indented, and angulated by the ribs: outfide, when 

 cleaned, frequently flreaked or rayed with brown : in- 

 fide of old fhells white, but in young ones of a deep cho- 

 colate-brown, with white ftreaks at the margin. Thefe 

 are the mofl effential varieties, and would certainly ap- 

 pear very diftinQ, were it not for the intermediate gra- 

 dations, that feem to run into each other, and which 

 has obliged later writers to confidcr them the fame. It 

 is proper, however, to remark, that thefe varieties (if 

 they are really fuch) have nothing to do with fize or age, 

 for we have found the deprelTed ones nearly as large as 

 thofe of a conic fhape; and which are alwavs more or 

 lefs ribbed ; whereas the latter are more properly flriat- 



ed ; 



