Oblong oval, nearly smooth, having a few concentric striæ, and some diagonal striæ at the ends of the valves; colour yellow or greenish brown; inside white.
2. Mactra. The Kneading-trough. Thirty-three species.
The name given to this genus was derived from the Greek word μακτρα, from its resemblance to a trough used for kneading bread.
In all species of this genus a similarity of colouring and form pervades the whole. In shape they are sub-triangular or oblong, with a smooth, striated, or transversely-ribbed exterior. In some species the valves gape at both ends, and in others at the anterior only. The most general colour is lilac, or white tinged with blue or yellow; some have purple rays on a brown ground.
A singularity in the form of the hinge of the Mactra distinguishes it from all other genera. It is of a triangular form, has a bent or angular compressed tooth on each valve, with a small oblique cavity on each side to which the ligament is attached. There are also two lateral teeth, one near the ligament and the other near the primary tooth. These teeth are thin and fragile; the primary tooth is sometimes indistinct, but the lateral teeth always exist.
The Mactra is found buried in the sand at a little distance from the seashore. Shell generally thin and brittle, covered with epidermis, of a triangular form, transverse, equivalve, inequilateral; beaks prominent; one compressed, folded, cardinal tooth, with an adjoining pit in each valve, projecting inward; lateral teeth thin, lamellous, entering, placed near the hinge; exterior ligament small; an interior ligament inserted in the cardinal pits; two muscular impressions, united by a narrow marginal tongue.
Mactra gigantea. M. Spengleri. M. striatella. M. carinata. M. straminea. M. Australis. M. violacea. M. fasciata. M. turgida. M. plicataria. M. rufescens. M. maculata. M. subplicata. M. triangularis. M. lactea. M. abbreviata. M. Helvacea. M. grandis. M. stultorum. M. maculosa. M. ovalina. M. alba. M. solida. M. castanea. M. rufa. M. squalida. M. Brasiliana. M. donacina. M. depressa. M. lilacea. M. trigonella. M. deltoides. M. crassatella.
Species in which the cardinal teeth are almost obsolete, in consequence of the enlargement of the pit of the ligament.