64 



ARCA. 



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 vi^ith 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, 

 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 Noce. With this the other species are con- 

 nected, not by form, but by the construction of 

 the hino;e. 



