GENERA OF SHELLS. 39 



(a.) JVo distinct angle at the posterior extremity of the car- 

 dinal line. 

 A. cygnea, anatina, sulcata, fragilis, calaracta, rubens, 

 crispata, uniopsis, Pennsylvanica, intermedia, suborbicu- 

 kta. 



(h.) A distinct angle at the posterior extremity of the car- 

 dinal line. 

 A. trapezialis, exotica, glauca, sinuosa, Patagonica. 



Iridina. Shell equivalve, inequilateral, trans- 

 verse ; beaks small, recurved, almost straight ; 

 muscular impressions as in anodonta. Hinge 

 long, linear, attenuated at the middle, tuber- 

 cular, almost crenulate throughout ; tubercles 

 unequal, frequent ; ligament exterior, margi- 

 nal. 



The principal difference between the anodonta and 

 ii-idina, consists in the tuberculated hinge of the latter. 

 The shell is rather thick, brilliant pearly, reddish, 

 especially internally, and iridescent, 



I. exotica. 



CHAMACEA. 



Shell inequitalve, irregidar, fixed; hinge with one thick 

 tooth, or loithout teeth; two separate and lateral mus- 

 cular impressions. 



The ligament is external, and sometimes sunk irreg- 

 ularly towards the interior; the hinge is somewhat 

 allied to the tridacnea. The shells are often lamellar 

 and spinous, their beaks always irregular, sometimes 

 large and contorted. They are attached to rocks, 

 corals, and often to each other. 



DicERAS. Shell inequivalve, adherent ; 

 nates conic, very large, divaricate, turned into 



