EXPLANATION OF TilC PARTS OF SHELLS. 17 



its being elevated, depressed, &c., depends much of the generic 

 and specific definition. Adanson, in his ' Natural History of 

 Senegal,' says that the external character of the spire varies 

 according to the plane they turn upon, which, he observes, is 

 either horizontal, cylindrical, conic, or ovoid. At the same time, 

 he admits that there are a great many intermediate forms which 

 cannot properly be defined. 



It must be remembered that many of the young shells have not 

 the same number of wreaths as the adults ; from which it would 

 appear, that the part of the animal nearest the apex never 

 increases in size. The number of wreaths cannot, at all times, 

 be depended upon. A full grown shell may, however, be known 

 from the outer 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 Up till full grown. 



Whorl is one of the wreaths or volutions of the shell. Plate II. 

 fig. 8, L ; and Plate III. fig. 10, L. 



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

 of the genus Planorbis, &c. PI. II. fig. 12, Z> ; and PI. III. fig. 

 5, S. 



A flat 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 Cypraea. 



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

 rates the wreaths or whorls from each other; it is sometimes crenu- 

 lated, undulated, or sulcated, and not unfrequently elevated or 

 projecting. Plate II. fig. 9, 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 take from the centre or apex ; and, in most instances, 



