64 
ARC A. 
A. Margin very entire, beaks recurved. (Plate 
VIII. Fig. 4.) 
B. Margin entire, beaks reflected. (Fig. 5.) 
C. Margin crenate, beaks recurved. (Fig. 6.) 
D. Margin crenate, beaks inflected. (Fig. 7.) 
Shell inequivalve ; hinge with numerous sharp 
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 eared ; and the 
surface is sometimes covered with a rough cuticle 
and beard. 
In many species the area lies flatly between the 
remote beaks ; it is horizontal or a little inclined, i 
and is covered with the ligament in a manner 
peculiar to this genus. In these the line of sepa- 
ration 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 No(e. With this the other species are con- 
nected, not by form, but by the construction of 
the hinge. 
