"Here are the remains of the same kind of substance which we saw in _Donax_ and in _Venus_. I think it is called the _ligament_." [Illustration: Plate 5.] "Very well remembered," continued her father. "The generic character of this very large portion of bivalves is, shell bivalve, usually with unequal valves: hinge without teeth, having a hollow cavity or sinus, and sometimes grooved. Here is a young common oyster, and, according to the rule of our present system, this shell, _Ostrea isognomon_, is of the same genus. (Plate 5.) The number of species is eighty-four, of which thirteen are British. The old shells of common oysters are often covered with _Serpula_, _Lepas_, and _Anomia_, and some kinds of corallines. [Sidenote: ANOMIA. MYTILUS.] "The next genus, _Anomia_, is remarkable for the thin, delicate, and almost transparent appearance of the shells. The valves are unequal, and frequently perforated near the apex; hinge toothless; in the flat valve, two bony rays. "_Anomia ephippium_ has a large perforation, through which the animal passes a ligament, and attaches itself to other substances. These shells are often to be found on oysters. (Plate 5.) Species thirty-two. "_Mytilus._ The principal characters are, shell bivalve, rough, often affixed by a thick byssus, or beard; hinge without teeth, with a hollow line extending lengthways. (Plate 3.) "The common muscle, _Mytilus edulis_, must be well-known to you, and also the fine polish that the shells will take when cleared of the rough exterior by artificial means, _Mytilus barbatus_ is not unfrequent on our shores; the colour is brown, and the shell is shaggy. Number of species, forty-nine. [Sidenote: PINNA.] "Our last bivalve genus is _Pinna_. The generic characters--shell bivalve, brittle, erect, gaping at one end, throwing out a byssus; hinge without teeth. (Plate 6.) "The _Pinna_ race are found plentifully in the Mediterranean, the Indian, American, and Atlantic oceans: the British seas afford three species. The genus is noted for producing a fine byssus, that is manufactured in Italy into various articles, as gloves. The animal is sometimes used as an article of food. "An ancient writer asserts that the _Pinna_ is attended by a crab, that finds a habitation in its shell, and repays the favour by giving notice, by a gentle nip, when a fish comes within reach; the _Pinna_ opens the valves of the shell, and secures the prey, which serves for the food of both. Now, Charles, you know the whole sense of the quotation-- "'The anchor'd pinna and his cancer friend.'" CHAPTER V. Third Division.--_Univalves._ "Father," said Lucy, the next time they met to pursue their study, "I think we have made ourselves familiar with the various hinges of the bivalve shells, which are becoming favourites with us; but from the variety of fine specimens which you have on your table, I see that we shall be much gratified in examining the univalve division." [Sidenote: ARGONAUTA.]