BIVALVIA. 



75 



ARCA,* Linnaus, 1758. 



PARALLELOPIPEDUM. Klein, 1753. 

 ANOMALOCARDIA. Id. (not Schum). 



CIBOTA. Browne, 1/56. 

 PECTUNCULUS. Adanson, 1/57. 

 ARCA. Linnaeus, 1758. 

 AMYGDALUM. Chemnitz, 1784. 

 DAPHNE and DAPHNODERMA. Poll, 1795. 

 TRISIDOS. Bolten, 1798. 

 CucuLL(EA. Lam., 1801. 

 ARCITES. Martin, 1809. 

 TRISIS. Ora, 1815. 

 CYPHOXIS. Rafinesque, 1819. 

 ARCACJTES. Schlot., 1820. 



BYSSOARCA. Swainson, 1820. 



NAVICULA. Blainv., 1825. 



RHOMBOIDES. 7c?. 



SCAPHULA. Benson, 1834, not Swainson. 



ARGINA? Gray, 1840. 



BARBATIA. 7c?. 



LUNARCA ? Id. 



LITHARCA. Id. 1844. 



SENILIA. Id. 



SCAPHURA. Id. 



ISOARCA. Munster, 1843. 



ANDARA. Gray, 1847. 



SCAPHARCA. 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 in 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 Bysso-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. 



