DIMYARIA. 145 



Cytherea Chione. — The Chione Cytherea. Plate 

 XVI. fig. 21. Somewhat heart-shaped, strong; covered 

 with a chestnut glossy epidermis, faintly wrinkled trans- 

 versely, longitudinally rayed ; with a cordiform depression 

 under the beak. Two and a half inches long. Inhabits the 

 British and Mediterranean seas. 



This genus is divided into three sections. — * The anterior primary 

 tooth, viit\y a striated cleft, or the margin dentated.— ** Anterior pri- 

 mary tooth not striated in its cleft, nor dentated in the margin, — 

 *•* The internal margins crenated or dentated. 



Genws 23.— ARTHEMIS.—Po/i.—EXOLETA._J?row;«. 



Generic Character. — Shell strong, orbicular, beaks much 

 turned to one side, beneath which is a short cordiform im- 

 pression ; hinge with three primary teeth, two of which are 

 contiguous, and the other divergent, which is broad in the 

 right valve, and cleft in the centre, to receive that of the 

 opposite valve, which is slender ; cartilage situated within 

 the outer surface of the shell; margins very broad, and 

 somewhat concave; edges very thick.* 



Arthemis orhiculata. — The Orbicdlar Arthemis. 

 Plate XVI. fig. 23. Orbicular, strong, with regular coarse, 

 close set transverse striae ; generally white or pale brown, 

 with sometimes three or four brown radiations emanating 

 from the umbo, and terminating in the margin ; inside white. 

 Two inches long. Inhabits the European seas. 



Genus 24.— CYPRINA Lamarck. 



Generic Character. — Shell equivalve, inequilateral, 

 obliquely heart-shaped, beaks obliquely bent ; hinge with 

 three unequal teeth, approximate at the base, and slightly 

 divergent above; lateral cardinal tooth remote, sometimes 

 obsolete ; ligament external, and partly sunk between the 

 beaks. 



» The name given to this genus by its founder, Poli, is that here em- 

 ployed. It subsequently received fi'om Captain Brown the name of 

 Exoleta, which, being an adjective, is inadmissible. 



L 



