ON MOLLUSCA. 269 
In a certain number of species the pieces are extremely 
small, and do not touch; in this case they might easily be 
taken for the imperfect shells of univalves, especially the 
first and last of the series. 
2. The lateral when they are in a greater or less number 
placed symmetrical on each side of the envelope of the ani- 
mal, a single one occupying the dorsal line. They may touch 
or exist only in a rudimentary state, but they never articulate. 
They may also vary considerably in form and size, and be 
more or less smooth or striated. 
These two groups of multivalve shells have been named 
dissivalves. 
3. The coronals or sub-coronals, first established by M. de 
Lamarck, when being disposed in a manner more or less re- 
gular round a common axis, they are solidly dovetailed toge- 
ther by the edges, so as to form a complete cavity, close or 
open inferiorly, and closed above by a small number of pieces 
of a form not very variable, the ensemble of which is termed 
operculum. 
The form, the number of the principal pieces, as well as 
that of the operculum vary, but the differences which they 
present do not require to be designated by peculiar terms. 
We shall now close this imperfect yet we fear tedious 
sketch, by a very brief account of the history of conchology. 
Aristotle, the first in this branch of the sciences, as in so 
many others, presents us, if not a systematic arrangement of 
shells, which was not his object, at least the basis of several 
divisions, which have been subsequently established. ‘Thus 
we find in his principal work that he has considered shells 
with a view to the principal relations under which we study 
them at present; that is to say, according to the number of 
the pieces of the shell, he divides them into monothyra or 
univalves, and into dithyra or bivalves. He then takes 
among the first the consideration of their turbinated or 
