ON MOLLUSCA. 253 
in a tube, or calcareous envelope, more or less developed, 
which some authors erroneously consider as another valve. 
They are always applied on the sides of the animal, and con- 
stantly in a relation more or less marked between themselves. 
Nevertheless, we should mention that this relation between the 
two pieces of a bivalve shell not being always evident, we may 
be sometimes led into error, and induced to regard as having 
belonged to an univalve, a piece or valve which belonged to a 
bivalve, as in the lingula, some species of camus, &c. 
The wnivalve shells are, on the contrary, a testa of a form 
extremely variable, sometimes even almost tubular, which 
covers the molluscous animal more or less, and may also be 
entirely concealed in the interior of its skin. 
The univalve shells may be considered under several differ- 
ent relations, which we shall just briefly mention here. 
1. Under the relation of the places in which they are found, 
or rather of the animals to which they have belonged, it has 
been thought proper to distinguish them into terrestrial, 
Jluviatile, and marine. But it must be owned that this dis- 
tinction is often very difficult, and that its importance has been 
exaggerated, as far as the use that the study of fossils may 
derive from it. 
The terrestrial univalve shells are generally rather thin ; 
their external surface, most frequently smooth, presents little 
but the striz of growth, and never any spines or asperities 
properly so called. The surfaces, both internal and external, 
are never nacreous. ‘Their aperture, always entire, has very 
often, at least in the adult state, and only in these species, its 
edges thickened in the manner of a pad, or more or less thrown 
out externally. 
The univalve fresh-water shells are also pretty generally of 
no great thickness ; they are sometimes furnished externally 
with some striz, and even with spines; and under the epider- 
mis, which is almost always thin, smooth, and of a very deep 
