Fig. 19. Hinge of Cardita sinuata.
Division b.—Siphonida Integropallialia.
1. The Chamidæ, represented by the reef-dwelling Chama.
2. The Tridacnidæ, whose sole genus Tridacna contains the largest specimen of the whole class of bivalves, the shells sometimes measuring two feet and more across.
3. The Cardiadæ, or cockles.
4. The Lucinidæ, in which the valves are nearly circular, and as a rule not very attractive in appearance, though the "Basket-shell" (Corbis) has an elegantly sculptured exterior.
5. The Cycladidæ, whose typical genus Cyclas, with its round form and thin horny shell, is to be found in most of our ponds and streams.
6. The Astartidæ, a family of shells having very strongly developed teeth, and the surface of whose valves is often concentrically ribbed.
7. The Cyprinidæ, which have very solid oval or elongated shells and conspicuous teeth ([Fig. 19]). The "Heart-cockle" (Isocardia) belongs to this family.