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. 



CARDITA,* Bruguiere. 1789. 



ANOMALOCARDIA (sp.) Klein, 1753. 

 ACTINOBOLUS. Klein, 1753. 



LlMNEA et LlMNODERMA (sp.) Poll, 1795. 



TRAPEZIUM (sp.) Humph., 1797. 

 BEGUINA. Bolten, 1798. 

 ARCTURUS. Humph., MS. 

 VENERICARDIA. Lam., 1801. 

 GLANS. Megerle, 1811. 

 ARCINELLA. Oken, 1815. 

 CARDISSA. Olten, 1815. 

 MYTILICARDIA. Elaine., 1824. 

 , CARDITAMERA. 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 

 costae ; 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. Kapbta, the heart. 



