PLATE LXXII. 



** These smaller ones," says Da Costa, " are the young Shells, 

 hut always with them are found old ones of double or treble the 

 size ; in every other respect like these, but proportionally larger and 

 stronger in tlieir several parts and work. 'Hie plaits or foldings 

 near the mouth are deep and very strong; the striae stronger and more 

 distinct ; the border round the mouth greatly turned outwards, very 

 broad, flat, tliick, milk white, and tlie sinuosities, jap-s or teeth, 

 within, are large, white, and very conspicuous ; some are bidentatcd, 

 and most of these old ones have eleven, and some even twelve spires. 



" From these circumstances, autliors run into confusion, by 

 making the difTerent growths different species. The accurate and 

 judicious Lister himself has formed two species, in his tit. 10. and 1 1. 

 on the ditFerence of the number of the spires and other sliglit parti- 

 culars. The several figures in Gualtieri are only varieties ; and the 

 bidens of Linne, Syst. Nat. p. 12iO. No. 649. and of Mr. Pennant, 

 Brit. Zool. No. 117. tab. 81, fig. 117. is apparently no other than an 

 old Shell, for such large and bidentatcd ones I have not unfrec[uently 

 found nestled with these common smaller Shells. 



*' Though tlie number of spires In a Shell Is a criterion, yet It Is 

 jiot an infallible one, for the number of spires vary in some species, 

 either from the growdis or sexes : in such cases the young Shells have 

 always a less number, and the males have their spires less numerous 

 than.the females. This very species is, perhaps, as strong an instance 

 of the difl-erence in the number of the spires as can be, for it is 

 found from six to twelve spires, as Linne has also noted in his 

 Fauna Suecica." 



Linns:us, and Qmelln in his last Systema Natur.-^, distinguish the 



