4 PARTS OF UNIVALVES EXPLAINED. 
thread-like line, which separates the volutions. Fig. 7. 
€e-e. 
Reversed Spire, is where the volutions revolve from right 
to left, the shell being held with the tip of the spire 
upwards. Fig. 12. d. 
There is also another characteristic, which is, that the 
aperture is situate opposite the left hand of the observer 
when viewed in front. Reversed, or sinister shells, are 
rare, in comparison to those that are dextral. The genera 
Clausilia and Physa are permanent examples of this confor- 
mation, as also Fusus perversus. Fig, 12. 
There are frequentiy reversed shells found among genera 
which are dextral ; but these are considered dusus nature. 
Depressed Spire, is exemplified in shells of the genera 
Planorbis, &c. Fig. 15. 
Involute Spire, is when the volutions are concealed 
within the body, or external volution, as exemplified in 
the shells of the genus Nautilus, Bellerophon, &c. Plate 
II. fig. 14. 
Aperture is the opening of the shell, by which the ani- 
mal protrudes itself, situated on the proper left side in 
dextral shells, plate I. fig. 7. f and in the right side in 
sinistral shells, fig. 12. e. ; 
Outer lip or Peristome, the left margin of the aperture, 
or final termination of the body volution. Fig. 12. c. and 
fic, 21.5: 
Inner or Pillar lip is that portion of the body bounded 
by the right side of the aperture, which is frequently 
covered by an enamelled thickening of testaceous matter. 
Pai 7 ee 
Columella, the lower front portion of the body volution, 
or external covering of the internal spiral supports of the 
spire, generally concealed by the inner lip. Fig. 7. h. 
Umbilicus, 2 small concave perforation or hole generally 
situate at the base of the body, and in some instances at 
the back of the reflected pillar lip. Fig. 14. a. 
Beak or Rostrum, a lengthened process, prolonged be- 
yond the body volution in many genera. Fig. 1.7% The 
peculiar structure of this prolongation forms an important 
feature in the discrimination of species. 
Canal or Gutter. The internal hollow or groove of the 
beak, or prolongation of the aperture at its base, formed 
by an extension of the outer and inner lips. Fig. 7. &. 
Chambers. Internal portions of the shells of the genera 
