"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. [Illustration: Plate 2.] [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. [Sidenote: TERMS--BIVALVES.] "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_.) "The _ligament_ is a membrane that connects the valves, and keeps the hinge in its proper place: it is always situated near the beaks. The ligament is very perceptible in the cockle, in _Pecten_, or scallop, in _Tellina_, &c. "The _beaks_ are the most pointed parts of the bivalve shell (Plate 3, _c_.); when the valves are closed, the line where they meet is called the _seam_. (Plate 3, _d_.) "The _anterior slope_ is that part of the shell where the ligament is placed, and is also called the _area_. (Plate 3, _e_.) The _posterior slope_, or _areola_, is the other side of the beaks. (_f._)