BIVALVIA. 75 
Arca,* Linnaeus, 1758. 
PARALLELOPIPEDUM. Alein, 1753. Byssoarca. Swainson, 1820. 
ANOMALOCARDIA. Id. (not Schum). Navicuta. Blainv., 1825. 
Crsota. Browne, 1756. Ruomsorpes. Id. 
PrctuncuLus. Adanson, 1757. ScapHuta. Benson, 1834, not Swainson. 
Arca. Linneus, 1758. Areina? Gray, 1840. 
AmyGpaLUM. Chemnitz, 1784. Barpatta. Id. 
DAPHNE and DapHNoDERMA. Poli, 1795. Lunarca? Id. 
Tristpos. Bolten, 1798. Lirnarca. Id. 1844. 
Cucutta@a. Lam., 1801. SENILIA. Td. 
ArcitEs. Martin, 1809. Scapnuura. Id. 
Trists. Oken, 1815. Isoarca. Munster, 1843. 
Cyrnoxis. Rafinesque, 1819. | ANDARA. Gray, 1847. 
Arcacitrs. Schlot., 1820. Scapuarca. Id. 
Generic Character. Shell inequilateral, generally equivalve, more or less quadrate 
or trapezoidal, sometimes closed; at others, with an opening at the ventral margin, 
thick and strong; externally striated or costated. Umbones distant. Hinge linear, 
with numerous close-set interlocking teeth. Ligamental area generally large and 
broad, with angular grooves. Palleal impression entire. 
Animal oblong, edges of mantle disconnected, simple or fringed, without siphonal 
tubes, a large and bent elongated foot with a groove capable of expanding into a disk- 
like form, and a gland at its base for the production of a byssus. Byssus compact. 
Animals of this genus generally spin a substance for their attachment, but many 
are found located im rocks or holes of shells. Some species have a considerable 
opening at the ventral margin, in consequence of which, those more strongly marked 
with that character were placed in a separate genus, under the name ysso-arca ; 
but in individuals of the same species, this opening is subject to great variation, being 
large in some, while in others it is nearly obliterated. 
M. Nyst has published a synoptical table of this genus, in which he has enume- 
rated 459 species, 162 of these are recent, whilst the others are extended through 
almost every Period, commencing with the Upper Silurian, and ranging through all 
the intermediate Formations: although some few of these, are probably, only varieties, 
such an extensive genus might naturally be expected to have a large geographical 
range; the recent species are found in all parts of the world, though not equally dis- 
tributed, nearly one hundred being found in the equinoctial regions; and its vertical 
range, is also very extensive, some being found under stones at low water mark, 
while others have been met with attached to rocks as deep as eighty fathoms. 
* Etym. So called from its Boat-like form. 
