EXPLANATION OF THE PARTS OF SHELLS. 25 



A fiat shell is figured in plate III. fig. 14. 



Involuted Spire, those shells which have their whorls, or 

 wreaths, concealed in the inside of the first whorl or body, as in 

 some of the Nautili and Cypraeae. Plate III. fig. 3, t; plate 

 VII. fig. 19; and plate VIII. fig. 21. 



Suture of the Spire, or whorls, is a fine spiral line, which 

 separates the wreaths or whorls from each other ; it is some- 

 times crenulated, undulated, or sulcated, and not unfrequently 

 elevated or projecting. Plate II. e e. 



Reversed, or Heterostrophe Spire, is when the volutions of 

 the spire revolve in the same manner as a common corkscrew, 

 or when the aperture is placed downwards^ the nature of the 

 spire runs upwards from the right hand to the left, plate III. 

 fig. 13. 



In some of the more depressed species of Helix, or Nautilus, 

 great attention is requisite in order to ascertain which is really 

 the upper side of the shell, for it is on that side the spiral turns 

 are to be taken from the centre or apex-, and;, in most instances, 

 this is to be determined by the oblique direction of the aperture 

 to the under part, where the lip rarely extends so far as on the 

 upper part. In fixed shells, such as Serpulas, there is no diffi- 

 culty, as the side which is sessile must be considered as the base, 

 or under part. Thus in the Serpula Lucida the fixed part is 

 sometimes very small, and the mouth protends spirally upwards, 

 in a contrary direction to the sun ; and therefore must be con- 

 sidered a reversed or heterostrophe shell, the same as if the vo- 

 lutions nearest the mouth had turned laterally upon the centre 

 or fixed ones. This shell, indeed, is most frequently found with 

 regular lateral volutions ; and though subject to great variety, 

 with respect to contortions, it invariably turns the aperture one 

 way. 



In some species of Nautilus, however, there can be no rule 

 to ascertain whether the shells are dextral or sinistral ; for when. 

 the aperture is exactly central, the lip embraces the body equally, 

 and the sides of the shell are similar, as in the Nautilus Calcar, 

 which cannot be defined. In others of that genus, as in N. Bec- 

 carii and Beccarii perversus, two shells, the principal distinction 

 of which is the contrary turn of their volutions, it is easily 

 determined by the convexity of the upper side, and, of course, 

 the aperture being placed somewhat beneath. 



