Fam. 2. Prasinidae.—Shell very small, umbones anterior, incurved, anterior side depressed, hinge-teeth replaced by dentiform projections of the lunule fitting into corresponding grooves. Recent. Single genus, Prasina.
Fam. 3. Ostreidae.—Heart generally ventral to the rectum, branchiae concrescent with the mantle, no byssus; shell inequivalve, fixed by the left valve, form irregular. Jurassic ——. Genera: Ostrea; Heligmus (Oolite), Naiadina (Cretaceous), Pernostrea (Jurassic).
Fam. 4. Pectinidae.—Byssus usually absent, mantle edge open, duplicated, folded back, with pallial ocelli; branchiae not concrescent with the mantle; shell with unequal “ears” at the umbo, hinge-teeth lamelliform, often obscure. Silurian——. Principal genera: Pedum, Chlamys, Hinnites, Hemipecten, Amussium, Pecten; Aviculopecten (Palaeozoic), Crenipecten.
Fam. 5. Limidae.—Mantle edge as in Pecten, tentaculate; shell sub-equivalve, eared, fixed by a byssus or free. Carboniferous——. Genera: Lima (Fig. [85], p. 179), Limea.
Fam. 6. Spondylidae.—Foot with a peduncular appendage, no byssus, numerous pallial ocelli; shell fixed by right valve, surface often very spinose, two cardinal teeth in each valve. Jurassic——. Genera: Plicatula, Spondylus; Terquemia (Lias).
Fig. 303.—Pecten pallium L., East Indies.
Fig. 304.—Spondylus petroselinum Sowb., Mauritius; on a coral.
Fam. 7. Dimyidae.—Shell ostreiform, fixed, hinge with or without symmetrical teeth, two muscular impressions. Single genus, Dimya (Tertiary).