86 



the hinge, render it necessary to constitute a distinct 

 genus of them. They are distinguished fromihe Area 

 by the orbicular compressed shape of the valves, (which 

 by age become extremely thick and ponderous, often at- 

 taining a very large size), and particularly by the hinge, 

 in which the teeth are less numerous, thicker, more se- 

 parated, and placed in an arched or curved position 

 round a part of the inner margin. The valves never 

 gape, nor does the animal attach itself by a byssus. — 

 The apices are small and distant, divided by an angu- 

 lar channel, in which the ligament is inserted. The 

 valves have often rayed longitudinal ribs, covered with 

 a velvet-like epidermis, and the interior margin crenu- 

 lated. The centre teeth of the hinge appear nearly ob- 

 solete or worn down. 



Longitudinal distant furrows, and often in addition fine 

 stria, either transverse or longitudinal. 



Pectunculus glycimeris Pectunculus angulatus 



pilosus stellatus 



undtilatus ' pal lens 



marmoratus violacescens 



scriptus zonalis 



pennaceus striatularis 



rubens nummarius 



