32 THE ELEMENTS 



is very flat. Plate 2nd, fig. 12th, D; 

 and plate 3d, fig. 5th, S. 



Involuted Spire, those shells which 

 have their whorls, or wreaths, con- 

 cealed in the inside of the first whorl 

 or body, as in some of the Nautilus 

 and Cyprsca. Plate 3d, fig. 3d, T; 

 plate 7th, fig. 19th; and plate 8th, 

 fig. 21. 



Suture of the Spire, or whorls, is a 

 fine spiral line, which separates the 

 wreaths or whorls ; it is sometimes 

 crenulated, in others sulcated, &c. 



Reversed, or Heterostrophe Spire. 

 As Mr. Montagu's definition of a re- 

 versed shell is very distinct and easily 

 understood, I shall here extract it. 

 " Every Conchologist knows, that 

 most convoluted shells turn one 

 way, and that there are few species 

 which are heteroclitical, or invariably 

 turn contrary to the usual manner. 

 Some indeed have doubted, whether 

 this is a permanent character ; there 



