ARCACEA. MOLLUSCA. ARCA. 95 



Family JlRCACEJl, Lam. 



Teeth small, numerous, disposed in a line along the hinge margin of each 

 valve. 



Genus ARCA, Lin. 



Shell transverse^ beaks separated by a diamond-shaped area for 

 the ligament ; series of teeth in a straight line. 



Arca pexa'ta. 



Shell oblong ; beaks prominent^ very oblique ; the ligamentary 

 space very narrow ; surface with about thirty-two radiating ribs, 

 covered with a shaggy^ broicn epidermis. 



Figure 60. 



State Coll., No. 186. Soc. Cab., No. 1949. 



Area pexata, Say; Journ. Jlcad. JVat. Sc, ii. 268. 



Shell thick and heavy, oblong, somewhat ovate ; very inequi- 

 lateral ; the beaks are ventricose and prominent, directed very 

 obliquely forwards, terminating in points which are nearly in con- 

 tact over the anterior termination of the series of cardinal teeth ; 

 at the other extremity of the series, the outline of the shell, which 

 is elsewhere regularly rounded, has an obtuse angle ; the ligamen- 

 tary area, or space between the beaks, is very narrow, scarcely 

 separating them. Surface with thirty-two to thirty-six radiating 

 ribs, rather broader than the channels between them ; these are 

 traversed by minute lines of growth, and interrupted by the more 

 distinct overlapping zones of increase. The whole is covered by 

 a thick, shaggy, fibrous epidermis of a dark brown-color, some- 

 times protruding from the interstices of the ribs in a fringe-like 

 manner. Interior white, the margin polished, and profoundly 

 scolloped by the alternate terminations of the ribs and grooves. In 

 front of the regular series of teeth are a few irregular pits and 

 prominences which fit into each other. Length 2| inches, 

 height 2~g inches, breadth 1| inch. 



The Arca pexata has never been found to the north of Cape 

 Cod. I have it from Martha's Vineyard ; it is not rare in Buz- 

 zard's Bay, and is common about Rhode Island. 



