338 Natural History of Molluscous Animals : 
What are vulgarly called Cornua Ammon/s are fossil multi- 
locular shells, and their preservation is so complete, that, from 
these remains, we can form some tolerably plausible notions 
of what has been the structure and habits of their animals. 
Further, the inmates of many univalves can close the aper- 
ture of their shells with a horny or calcareous plug attached 
to the dorsal part of the foot, and termed the operculum. 
These are said to be operculated, while those which have no 
protection of this sort are non-operculated shells. The for- 
mer, in the opinion of some naturalists, are the links which 
connect the univalves to bivalves, the operculum being con- 
sidered analogous to the second valve, and Blainville indeed 
goes so far as to call them sub-bivalves.* This is surely 
str aining analogies, and seeking them in things between which 
there is) no mutual resemblance; but in fact. ‘the opinion is a 
part of a theory relative to the mutation of one organ into 
a A which has been brought forward and illustrated at 
sat length by some French “phy siologists, and too hastily 
sloped by some of our own countrymen. We will probably 
have a future opportunity of discussing this doctrine, which, 
I may here remark, appears to be the offspring of metaphy- 
sical subtlety, and as likely to lead to error in regard to the 
functions of parts, as to guide us to a knowledge of their true 
uses. 
To return from this digression, I remark that univalve 
shells are again distinguished into such as have the margin of 
the aperture entire, and into such as have it notched, channel- 
led, or more or less lengthened into a canal. ‘The latter are 
ad to be canaliculate, or emarginate, or effuse, and, as will 
be afterwards shown, differ from ee first in the ais of 
their animals, so that the character is of greater importance 
than is at first obvious. ‘These distinctions and terms it is 
necessary to remember, as I shall have frequent occasion to 
use them in our future correspondence. 
However varied in external character, shells differ very 
little in their chemical composition. They all consist of car- 
bonate of lime united to a soft albuminous matter, and any 
variation that occurs in different shells is merely in the rela- 
* Manuel de Malacologie et de Conchiliologie, p. 229. In another 
place, however, he more correctly observes, “ Quelques auteurs, et entre 
autres Adanson, Yont regardée comme l’analogue dune des valves d’une 
coquille bivalve, mais évidemment a tort; car sa position, par rapport au 
corps de lanimal, m’indique aucune analogie,’ &c. (p.103.) — [ Some 
authors, and amongst others Adanson, have regarded it as the analogue of 
one of the valves of a bivalve shell, but evidently wrongly ; for its position 
with relation to the body of the animal does not indicate any analogy.” | 
