Natural History of Molluscous Ariimals, 335 



Art. VIII. A7i Introduction to the Natural History of Molluscous 

 Animals, In a Series of Letters. By G. J. 



Letter 5. On the Forms, Composition, and Structure of Shells. 



Sir, 

 Molluscous 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 testaceous Mollusca; 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 arbitrary. In 

 several 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 multi- 

 valve, bivalve, and miivalve ; 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 Linnaeus. {Jig. 86.) 



The bivalves are very numerous, and belong 



exclusively f to a distinct order of molluscous 



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 (Xepas Xw/.) amongst the Mol- 

 lusca. 



f So at least says Lamarck ; but the shells of some Entomdstraca appear 

 to be true bivalves. 



z 4 



