This genus is easily known by its resemblance to the hull of a ship; the
hinge is peculiar, being composed of numerous sharp teeth alternately
inserted between each other. The Arca of Linnæus was divided by Lamarck
into the four genera that compose this family, each possessing a strong
distinctive character. All the shells of this family are found in the
sea at a little distance from the shore; they are covered with a dark
greenish lamellar or velvet-like epidermis, frequently ending in a deep
fringe at the margin.
Shell a little varied in form, but most generally elongated, and more or
less oblique at the posterior extremity; sometimes very inequilateral;
summits more or less distant, and a little recurved forward; hinge
anomalous, straight, or a little curved; long, and formed by a line of
short vertical teeth decreasing from the extremities to the centre;
ligament exterior, broad; sometimes the margin is crenated.
Arca tortuosa.
A. semitorta.
A. Noæ.
A. tetragona.
A. umbonata.
A. sinuata.
A. avellana.
A. cardissa.
A. ventricosa.
A. retusa.
A. sulcata.
A. ovata.
A. Helbingii.
A. scapha.
A. barbata.
A. fusca.
A. Magellanica.
A. Domingensis.
A. lactea.
A. trapezina.
A. pistachia.
A. pisolina.
A. cancellaria.
A. callifera.
A. irudina.
A. bisulcata.
A. Indica.
A. senilis.
A. antiquata.
A. rhombea.
A. granosa.
A. auriculata.
A. Brasiliana.
A. corbicula.
A. squamosa.
A. Cayennensis.
A. inequivalvis.
_A. Noæ._ Noah’s Ark. Pl. 10, fig. 5.
Species boat-shaped, oblong, striated transversely and ribbed
longitudinally; umbones remote and incurvated; margins entire and
gaping; hinge straight; whitish, with divergent zigzag chestnut stripes;
inside bluish white.
_A. tortuosa._ The twisted Ark.
A rare species; shell elongated, close, twisted; hinge completely
straight.
_A. barbata._ The bearded Ark.
Species with the hinge straight, not hollowed or not gaping inferiorly,
and of which the muscle is not adherent.