CHICUM. 323 
exotic Vermetus, both as regards animal and shell, cannot be 
placed satisfactorily in any of the groups between the Helicide 
and Pyramidellide. The reasons for this arrangement are so 
fully explained in the subjomed prologue on Cecum, as to 
require no further immediate comment; and those relating 
to Turritella will also be found in. our notes on that genus. 
CAICUM, Fleming. 
This singular genus has long been the reproach of concho- 
logists, and the pons asinorum of this branch of science ; 
the greatest names have failed to see its true position ; it has 
run the gauntlet from Dentalium, &c. of the Mollusca, to 
the Foraminifer Orthoceras. We do not attribute the satis- 
factory solution of the problem to any hypothesis of ours, but 
to time and opportunity, which, almost agamst hope, pre- 
sented the living objects of our research. The animal, in 
essentials, whether it be considered an aberrant Vermetus, 
Siliquaria, or Cecum, must in all probability follow Trochus, 
with Turritella, as I think these genera will prove herm- 
aphrodites. 
At one time we were almost inclined to substitute—but 
our reverence for priority prevailed —Vermetus tor Cecum, 
consequence of the animal and shell of the two being so 
much alike. The shell of C. glabrum, when very young, like 
Vermetus, forms spiral turns before it commences the elonga- 
tion; these in Cecum, as well as many of the subsequent 
portions, as the animal grows and withdraws itself therefrom, 
fall off, the cylinder above them having been previously 
plugged up; Vermetus does the same, but remaims with the 
terminal spiral attachment. I also believe that the other 
species, the C. trachea, has a right to claim the Skenea rota of 
authors, or an object closely similar, for its spiral turns, in a 
very young state; in it the three or four epochs of growth are 
as distinctly visible as in the elongations of the cylinder of 
the C. trachea at maturer ages. Also, under the microscope, 
the apex of the young C. glabrum, the Serpula incurvata of 
Montagu and Walker, agrees with that of the so-called 
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