"The shells of this genus are usually thin, brittle, and remarkably light. Mactra stultorum is a common species. ([Plate 4.])
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."
"Learn first to know the general appearance of Venus, and remark especially the teeth. You may also bear in mind that the beaks are almost always turned to the posterior slope, and from the ligament. The area and areola are also very conspicuous: the area is generally large, and differently coloured to the disk. Venus Paphia is pretty. ([Plate 4.]) The spinous species, V. Diòne, is more beautiful, and is the only shell of the genus that has spines. The brown Venus, V. chionè, is very smooth and polished; both species are frequent in collections. The British shells of this genus are neither numerous nor very beautiful.