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.
_M. gigantea._ The gigantic Mactra.
Species in which the cardinal teeth are almost obsolete, in consequence
of the enlargement of the pit of the ligament.
_M. stultorum._ The fool’s Mactra. Pl. 9, fig. 6.
Species in which all the teeth are very large, lamellous, striated
longitudinally; colour reddish brown.
_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.