EXPLANATION OF THE PARTS OF SHELLS. 29 



lip, which has generally an unfinished appearance in young 

 shells. Indeed, in all the land and fresh-water shells, it is 

 a distinct criterion, as they are never complete in the form 

 of the outer-lip, till full grown. 



Whorl is one of the turns, wreaths, or volutions of the 

 shell. Plate II. fig. 8, L; and Plate III. fig. 10, Z. 



Depressed Spire is when the spire is very flat, as in the 

 shells of the genus Planorbis, &c. Plate II. fig. 12, d; and 

 Plate III. fig. 5, s. 



A flat shell is figured in Plate III. fig. 14. 



Involuted Spire, in 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 Cyprtcse. Plate III. 

 fig. 3, t; Plate VII. fig. 19; and Plate VIII. fig. 21. 



Suture of the Spire, is a fine spiral line, which separates 

 the wreaths or whorls from each other; it is sometimes 

 crenulated, undulated, or sulcated, and not unfrequently 

 elevated or projecting. Plate II. fig. 9, e e. 



Reversed, or Hetero strophe Spire, is when the volutions 

 of the spire revolve in the same manner as a common cork- 

 screw, or when the aperture is placed downwards, the suture 

 of the spire runs upwards from the right hand to the left; 

 as in Plate III. fig. 13. Such shells are also called Sinis- 

 trose, or turned to the left. 



In some of the more depressed species of Helix, or Nau- 

 tilus, 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 Serpulae, there is no difficulty, as the side 

 which is sessile must be considered as the base, or under 

 part. Thus, in the Serpula lucida the fixed part is some- 

 times very small, and the mouth protends spirally upwards, 

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

 considered a reversed or heterostrophe shell, the same as if 

 the volutions nearest the mouth had turned laterally upon 



c3 



