"The generic characters are chiefly these: shell compressed towards the anterior slope, teeth of the hinge mostly three, the lateral ones smooth, in one valve. Two or three small species are common on our coasts. I should also observe that there is a convex fold on one valve and a concave fold upon the other. Many of the _Tellìna_ genus are found buried in the sea-sands. [Sidenote: CARDIUM. MACTRA.] "_Cardium._ Generic character: shell equivalve, convex, ribbed, striated, or grooved, the margin toothed: hinge with two teeth near the beak, and a lateral one on each side: fifty-four species. "Observe how the beaks of this common _Cardium_, cockle, turn inwards, and the bosses project. Another striking character is the ribs, that are generally longitudinal, and not concentric or transverse, as in _Tellìna_, and, as you will see, in _Venus_. _C. aculeatum_ has small spines on the valves; _C. costatum_, the ribbed cockle, is one of the finest species of this genus, and _C. cardissa_ is a beautiful shell. (Plate 4.) The common cockle is _Cardium edule_. _Mactra._ Generic character: shell bivalve, unequal sided, middle tooth of the hinge complicated, with a small hollow on each side, and lateral side-teeth: thirty-seven species. "The shells of this genus are usually thin, brittle, and remarkably light. _Mactra stultorum_ is a common species. (Plate 4.) [Sidenote: DONAX. VENUS.] "_Donax._ Margin of the shell often crenulate, the anterior slope very obtuse; hinge with two cardinal teeth, and one lateral tooth. "The most striking characteristic of _Donax_ is the broad, thick extremity of one end, gradually lessening towards the other. A rich purple tint is very frequent in these shells. _Donax denticulatus_ and _trunculus_ are common British examples. (Plate 4.) You must remark the ligament of _Donax_, which is exterior. "Our next genus ranks highest for beauty among the bivalves, and takes its name from the goddess _Venus_. The species amount to one hundred and sixteen in the Linnæan system, but other authors have formed several new genera. "Shell bivalve, having the lips incumbent on the anterior margin; hinge with three teeth, all approximate, the lateral ones diverging at the lip. "I am afraid," said Lucy, "that we shall find this genus very difficult: I wish you would tell us the new genera that have been formed out of it."