84 



separated by the angular area or channel of the liga- 

 ment, which is always external. Hinge in a right line, 

 without ribs at the extremities as in the Cucullaea, and 

 furnished with numerous acute teeth alternately in- 

 serted between others on the opposite valve: in many 

 species the valves when closed gape in the centre, oc- 

 casioned by the wide flexuous curve of their outer 

 margins, and sometimes one valve overlaps the other. 

 They are said to spin a byssus, and are covered with a 

 lamellar or velvet-like epidermis, frequently ending in a 

 deep fringe at the margin. Valves longitudinally ribbed, 

 imbricated, smooth, granulated, or finely striated. 



It might perhaps be advisable to distinguish the 

 species of Area which gape, and certainly spin a bys- 

 sus, from those which have their valves closed at eve- 

 ry part, and probably are never affixed by a byssus to 

 marine bodies. 



Superior Margin not crenulated within. 



Area tortuosa A rca sinuata 



semitorta avellana 



Noae cardissa 



• • • • letragona ventricosa 



• • • • umbonata • ■ • • retusa 



