"Thus you perceive that while you are obtaining knowledge in one science, you are preparing yourselves for making advances in another, most interesting and wonderful. You, Charles, who are likely to become a traveller, will perhaps in future years find the advantage of my present brief lessons.
"I shall first endeavour to make you acquainted with the system of Linnæus; it is easily learned, and you should be familiar with it, as it is still adopted by some writers on conchology.
"But in order to understand my instructions, you must have a clear idea of the terms that I use in describing a shell; now, therefore, give me your attention while I explain some of those terms to you.
[Sidenote: TERMS FOR PARTS OF MULTIVALVES.]
"To begin with the first division, _Multivalves_. There is a group of _Lepades_, it is the species called goose-barnacle, of which so many strange and silly tales have been told in former times. (Plate 2, _Lepas anatifera_.) This species is furnished with a kind of stem, like a bladder, and is called the _peduncle_, (_c_) and is fastened to other bodies. The _feelers_ (_d_) are feathery projections, which the animal keeps in continual motion, for the purpose of catching its food. Here is a group of another kind; (Plate 2, _Lepas tintinnabulum_;) these are without a peduncle, and are called _sessile_. The _base_ (_a_) is that part of the shell by which it is fixed to other bodies: (_a_) the _operculum_ is formed of four small valves on the summit. (_b_).
"In the shells of the second division, _Bivalves_, we shall find a greater number of parts. _Valves_ are the different pieces that compose a shell. When both the valves are alike in form, the shell is called _equivalve_: when the valves are different in the same shell, it is called _inequivalve_. _Mya_, _Solen_, _Tellina_, are equivalves: _Ostrea_, _Anomia_, _Pinna_, &c. are inequivalves.
"The _hinge_ is formed by the teeth of one valve inserting themselves between those of the other valve, in some genera; in others, by the teeth fitting into the _cavities_ of the other valve (Plate 3., _a_.) When the teeth are placed in the centre of the hinge they are called _cardinal teeth_. _Lateral teeth_ are situated on the sides of the valves, and are generally long and flat, sometimes hollow. Some hinges are straight, others curved. Here is the hinge of _Arca_, furnished with many small teeth. (Plate 3, _b_.)