EXPLANATION OF THE 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 lip till full grown. 
_ Whorlis one of the wreaths or volutions of the shell. Plate II. 
fig. 8, 1; and Plate III. fig. 10, Z. 
Depressed Spire is when the spire is very flat, as in the shells 
of the genus Planorbis, &c. PI. II. fig. 12, DY; 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 Cypreza. 
Suture of the Spire, or whorls, is a fine spiral line, which sepa- 
rates the wreaths or whorls from each other; it issometimes 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, 
a 
od 
