64 



ARC A. 



A. Margin very entire, beaks recurved. (Plate 

 VIII. fig. 4.) 



B. Margin entire, beaks reflected, (fig. 5.) 



C. Margin creniitc, beaks recurved, (fig. G.) 



D. Margin crenate, beaks inflected, (fig. 7.) 



Shell iuequivalve ; hinge with numerous sliarp 

 teeth alternately inserted. These teeth are most 

 frequently placed in a longitudinal direction on 

 the hinge. Shape various, oblong, rhombic, sub- 

 cordate, lenticular, subovate or cared ; and the 

 surface is sometimes covered witii a rough cuticle 

 and beard. 



■ In many species the area lies flatly between the 

 remote beaks; it is horizontal or a little inclined, 

 and is covered with the ligament in a mrnincr pe- 

 culiar to this genus. In these the line of separa- 

 tion at the hinge is perfectly straight. 



The name of the genus is taken from one of its 

 species, which is supposed to resemble Noah's ark, 

 Area Nocc. With tliis the other species arc con- 

 nected, not by form, but by the construction of 

 the hinge. 



