Natural History of Molluscous Animals. 335 
Art. VIII. An Introduction to the Natural History of Molluscous 
Animals. Ina Series of Letters. By G. J. 
Letter 5. On the Forms, Composition, and Structure of Shells. 
Sir, 
Mot.uscovus animals, in one point of view, may be divided 
into two classes ; those, viz., which have, and those which have 
not, a shell to cover and protect the body. The latter are 
said to be naked, and the former festacecus Mollisca; but 
however discriminative and important the character which 
separates them may at first appear, it is really, in a systematic 
view, one of trivial consequence, and perfectly arbitr: ary. In 
eevecall testaceous species the shell is too small to contain the 
body of the animal, while many of the naked tribes possess 
a shell which, however, is concealed under the folds of the 
skin, and is protective only of the internal viscera. The 
system which separates and places in distinct orders the naked 
slug and the shelled snail must be artificial; and if for the 
present I adopt the distinction, it is because I now wish to 
give you some account of the structure of shells, and of the 
manner of their formation ; for I would not that you should be 
familiar with their varied forms, and yet be ignorant of the 
mode in which they are constructed. 
Conchologists arrange shells under three orders, the madt7- 
valve, bivalve, and univalve ; or, to reverse the series, into 
shells composed of one, two, or more than two, pieces. 
After removing such species as belong to animals * of a 
different category, there remain very few multivalve shells. 
These have their valves connected by a soft coriaceous mem- 
brane, and they are laid, in an imbricated manner, along the 
back of the animal, which, in general outline, resembles a 
wood-louse. They constitute the genus Chiton 
of Linneeus. (fie. 86.) 
The bivalves are very numerous, and belong 
exclusively + to a distinct order of miolluscous 
animals. These shells are formed in general 
of two pieces only, placed in apposition, equal or nearly so in 
size, and commonly alike in form. ‘The pieces or valves are 
connected at their beaks by an elastic ligament of a peculiar 
structure, which forms a hinge on which the valves open and 
shut. In a few instances there is no hinge of this sort; but a 
* We do not include the Cirripedes (Lepas Lin.) amongst the Mol- 
lisca. 
+ So at least says Lamarck ; but the shells of some Entoméstraca appear 
to be true bivalves. 
ZA 
