ON MOLLUSCA. 263 
English ; and as this sort of pillar, when the base of the shell 
is very oblique, is often lengthened to its anterior extremity, 
it is it which in this case forms entirely the left edge of the 
aperture, in consequence of which that sometimes receives 
the name of columellaria. 
This column or pillar is called pointed when it is termi- 
nated anteriorly in a point, as in lyra; truncated when it ap- 
pears to have been cut, as inthe agathini ; salient or project- 
ing when it forms an elongation in front of the shell, as in 
the terebellz ; spiral when the part thus extended is twisted 
like a gimblet ; folded when a greater or less number of ob- 
lique folds are perceptible there proceeding from its torsion, 
as in the volute; and it is said to be provided with a pad 
when towards its extremity it presents a swelling more or less 
considerable, and transverse, as in some cerithia. 
Externally, or to the left of the termination of the pillar, is 
often seen a hole, or rather a cleft, more or less deep, of a 
form somewhat variable, and which especially exists in young 
subjects. ‘This is the umbilicus, whose formation we have 
explained above. From the presence or absence of this hole 
proceeds the distinction of shells into wmbilical or non-umbi- 
lical. We call the wmbilicus, consolidated, or subconsolidated 
when in the full-grown shell it is covered by a sort of calca- 
reous deposition, called a callosity, but this likewise exists 
underneath. If it exhibits grains projecting in its circum- 
ference it is called crenulated ; dentated if it is accompanied 
with one or many teeth, as in the turbo pica; canaliculated 
when at the interior it presents a spiral gutter, as in some 
turbines and many cerithia. 
After having thus successively viewed the univalve shells 
in their ensemble, and at their external surface, we will now 
consider their interior and orifice. ‘ 
The cavity of a shell may be entirely Patines by the 
animal, or the part which -is occupied may be separated from 
