"Multivalves may be divided into two kinds, the _pedunculated_, or those fixed to other bodies, as rocks, stones, planks, &c.; or _free_, as the _Chiton_ and _Pholas_. "Our first genus is _Chiton_. The shell is easily known. The fixed character is, many valves placed over each other along the back. I have already noticed the membrane which connects the valves, which is also a permanent character. It is elastic: the sides are either scaly, as in _C. squamòsus_, (Plate 1,) and hairy, or spinous. The species are determined by the margins. Some of the Chiton genus are common upon our own coasts; they are frequently found among seaweed and stones, rolled up like a ball. _C. fasciculàris_ and _C. lævis_ are British; there are some other British species. The animal adheres to rocks, like the _Patella_, or limpet. The number of species forty. "Second genus, _Lepas_. Shell multivalve, fixed at the base; valves erect, or _upright_. "Observe how much the situation of the valves differs in _Chiton_ and _Lepas_. It is scarcely possible to mistake the one for the other. The feathery tentacula, or feelers, of _Lepas anatifera_ are worthy notice, and in a state of motion must be yet more beautiful. (Plate 2.) The common acorn-shell, _L. balanus_, is to be seen very frequently upon the shells of muscles, oysters, periwincles, whelks, in large groups. I see that you are examining the different appearance of the pedunculated and the sessile _Lepades_. Linnæus made two divisions; later writers have separated them into several distinct genera, which will be noticed when we attend to Lamarck's system. The species are forty-five, of which several are found on the British shores, as _L. tulipa_, _L. diadema_, _L. tintinnabulum_, _L. balanus_, _L. anatifera_, &c. The Indian, American, and Atlantic oceans afford numerous species. [Sidenote: PHOLAS.] "Third genus, _Pholas_. Generic character: shell bivalve, gaping or divaricated, with several smaller hinges situated upon the hinge; hinge recurved, with an incurved tooth." "Father," said Charles, "I must say that the _Pholas_ shell is very unlike those of the multivalve division: I think it should rank with bivalves." "So many conchologists have judged; nevertheless it possesses more than _two_ valves, and, according to the system, it must be forced into the division of multivalves. "The _Pholades_ are found in company, but each individual occupies a distinct habitation, which the animal excavates for itself, either in rocks, in wood, coral, or sponge; but the finest specimens are usually to be seen in chalk. In proportion as the animal increases in size, it enlarges the cavity in which it is stationed. The animal is supposed to effect this operation by means of a corroding fluid that is secreted in the body, and which it has the power of ejecting upon the substance into which it has entered. [Sidenote: PHOLAS.]