EXPLANATION OF THE PARTS OF SHELLS. 17 



closes nicely up : as exemplified in the operculum of the Turbo, 

 and is of a hard stony appearance. Plate III. fig. 4. 



Spire consists of all the whorls of the shell, except the lower 

 one, which, as before observed, is termed the body of the shell. 

 Plate II. fig. 12. D; and Plate III. figures 7 and 8. B D. 



The spire is a prominent feature of the Univalve ; and upon 

 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 nnmber 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-lip till full grown. 



Worl is one of the wreaths or volutions of the shell. Plate IL 

 fig. 8. L ; and Plate III. fig. 1 0. L. 



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

 of the genus Planorbis, &c. PI. II. fig. 12. d; 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 Cyprse. 



Siitiire 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. 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 ths 



