"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. [Illustration: Plate 4.] [Sidenote: SPONDYLUS.] "_Spondylus._ Valves unequal, rough; hinge with two recurved teeth, with a hollow between them; shell sometimes eared. (See Plates 3 and 5.) "I think," said Lucy, "that the English name, _thorny-oyster_, is not very suitable: it is more like a scallop; but it differs from both in having two strong teeth in the hinge, and I observed this morning that neither the oyster nor the scallop have any hinge." "So that was the object you had in view," said Charles, "when you were so quietly handling those shells in the kitchen: I confess I could hardly help laughing; and now my father will say that _you_ are '_Eyes_,' and I the '_No Eyes_,' of 'Evenings at Home.'" "Perhaps I might have made the observation," replied Mr. Elliot; "but you have reproved yourself, which is far better. "_Spondylus_ can scarcely be mistaken from any other bivalve shell. The species _gæderopus_ is remarkable for its projecting beak; the surface is rough, with either tubercles or spines. Some authors reckon only four species, others thirteen. The _Spondyli_ are frequently found attached to rocks at some depth in the ocean. The animal is eaten on the coasts of the Mediterranean. We have no British _Spondylus_. [Sidenote: CHAMA. ARCA. OSTREA.] "_Chama._ Shell thick; hinge with a thick tooth, sometimes crenate, obliquely inserted into a corresponding channel. (Plate 5.) The shells of this genus vary greatly, which you will perceive upon comparing _C. gigas_ and _C. cor_. (See Plate 9, _Isocardia cor_.) The _Chama_ genus is usually ribbed, foliated, or scaly. _C. Lazarus_ is a beautiful species: _C. cor_ is a British species, and the only one. The whole number is twenty-five. "Here is _Noah's-ark_, an example of the genus _Arca_, and is found on our own coasts. The long hinge beset with sharp teeth, inserted into each other, renders the genus sufficiently marked; but in some species the hinge is curved. The form varies exceedingly. The number of species is forty-five. (Plate 5.) "_Ostrea._ In this well-known genus we lose sight of the _toothed_ hinge. Take that _Pecten_, or scallop, which belongs to one division of _Ostrea_ in this system, and tell me what holds the valves together. Charles is silent; what says Lucy?"