[Sidenote: MULTIVALVES, BIVALVES, UNIVALVES.] "Probably because you are better acquainted with plants," observed his father, Mr. Elliot, who had just entered the room: "however, the great naturalist, to whom you are indebted for your knowledge of plants, did not consider shells as objects beneath his attention." "You mean Linnæus," said Lucy; "then he, I suppose, separated shells into the three different divisions--_Multivalves_, _Bivalves_, and _Univalves_." "You are right, Lucy," replied her father. "Pray show me some bivalve shells," said Charles; "I want to know their forms. A _bivalve_ is a shell with two openings, as I should imagine: yes, I see that I am right, for you have given me an oyster and a cockle." "Here are also _Venus_, _Tellìna_, _Donax_, _Arca_, and _Pinna_," observed Mr. Elliot, "all very easy to distinguish." "'The anchor'd pinna and his cancer friend,'" repeated Charles. "So the _Pinna_ is a bivalve; but what has _Venus_ to do with the matter?" "That is very easy to understand," said Lucy: "the genus called by her name is remarkable for beauty." "Now, Charles," said Mr. Elliot, "do you clearly comprehend the verse that you have just repeated?" [Sidenote: THE PINNA AND THE CANCER.] "I have heard that the _Pinna_ is a shell-fish, attended by a _crab_, 'his cancer friend;' but why it is called _anchor'd_ I do not know, but _cancer_ is Latin for crab." "Here is a species of _Pinna_," said his father, opening a cabinet; "and these silken threads are the means by which it fastens itself to the rocks. The animal is provided with a long foot, with which it draws out the threads, or _byssus_. The _Pinna_ is sometimes called the silk-worm of the sea. Lucy, do you know a univalve shell?" "Oh yes, many!" replied Lucy, "here are rock-shells, cowries, limpets, and cones. I know the difference between a cowry and a cone; but I am not yet acquainted with a multivalve shell--will you show me one?" "_Chiton_, or coat of mail, is a good example," said Mr. Elliot; "_Pholas_ is another genus of the same division; it has the appearance of a bivalve. In the _Chiton_ are several _lamina_, or plates, which are connected by a membrane while the living animal is in the shell; the membrane is pliant, and the inhabitant has the power of contracting itself into a ball, when it would avoid injury, like the insect millepes, that we find under stones in damp places. (Plate 2.) The curious barnacle-shell, _Lepas_, is another multivalve." (Plate 1.) "I think we cannot have a more agreeable pursuit for our leisure hours," said Lucy, "than the study of shells. One can bring them out or remove them so easily, that they can cause little inconvenience, which garden-pots often do in town; and then the plants are almost sure to die, whatever care I take of them." [Sidenote: LAMARCK'S SYSTEM.]