M. solida. The solid Mactra.
Species thick, solid, without epidermis; lateral teeth finely striated.
M. trigonella. The three-cornered Mactra.
Species in which the lateral teeth are almost obsolete; exterior surface smooth.
M. triangularis. The triangular Mactra.
Species very small, strong, opaque, white; inside white; margin strongly crenated.
M. crassa. The thick Mactra.
Species very thick, solid, striated longitudinally; the cardinal teeth obsolete, or almost so; the lateral very thick, very close, and reflected; an external ligament as well as an internal one.
3. Crassatella. Eleven species.
May be easily known from the Mactra and Lutraria, as the valves, when closed, fit exactly, and do not gape. It is remarkable that all the living species contained in this genus only exist in the seas of Australasia, while at least seven species in a fossil state are found in France.