164 MOLLUSCA FROM THE CRAG. 
right, while that of the fossil from the Crag, now admitted by all conchologists as 
only a variety, turns generally to the left. In my first examination of the recent 
species, there appeared a difference of form in the impressions by the adductors, which, 
however, further observation proved to be no permanent character; the shape of 
these muscle marks being considerably modified by the outward form of the shell, and 
like the oyster, or any other adherent species of the Bivalvia, is more subject to dis- 
tortion by the position it has chosen to fix itself, than is ever partaken of by those 
species which are free, and the muscle marks undergo an elongation or contraction, 
conforming themselves, in that respect, to the varying properties of the shell, and 
showing that these are no more to be relied upon than are the outward forms of 
the shells themselves. 
Carpita,* Bruguiere. 1789. 
ANOMALOCARDIA (sp.) Kein, 1753. 
Actinopouus. Klein, 1753. 
Linea et Limnoperma (sp.) Poli, 1795. 
TRAPEZIUM (sp.) Humph., 1797. 
Breuina. Bolten, 1798. 
Arcturus. Humph., MS. 
VENERICARDIA. Lam., 1801. 
Guans. Megerle, 1811. 
ARCINELLA. Oken, 1815. 
Carpissa. Oken, 1815. 
Mytinicarpia. Blainv., 1824. 
CaARDITAMERA. Conrad, 1838. 
AGARIA. Gray, 1840. 
Gen. Char. Shell regular, thick, and strong, equivalve, inequilateral, suborbicular 
or transverse, closed, generally covered with more or less elevated, thick, and distinct 
coste ; hinge furnished with two teeth, one short, placed near the umbo, the other 
oblique, sub-marginal, prolonged towards the posterior. Impression of the mantle 
without a sinus, those by the adductors deeply seated. Ligament external. 
Animal of the form of the shell, with the edges of the mantle disunited, and 
without projecting siphons ; foot small and elongated: buccal opening furnished with 
three or four pairs of tentacles. 
Shells constituting this genus are ornamented upon the exterior with ribs radiating 
or diverging from the umbo, and are more or less elevated, becoming in some species 
nearly obsolete, and the shells are always thick and strong. 
In the recent state, as a Genus, it has an extended geographical range, one species 
being found on the Coast of Norway, while others inhabit the seas of New Holland, 
the Indian Ocean, China Seas, shores of Western Africa, and the East Coast of 
* Etym. capéa, the heart. 
