PECTUNCULUS. 97 



N. nitida : rather triangular, tapering at the hinder end ; 

 epidermis shining. 



II. Leda differs from the other animals of the genus in 



having distinct siphonal tubes. The shell differs from 

 that of Nucula in having the hinder end lengthened. 

 That of iV". pygmma is smooth and short ; that of N. 

 caudata is longer and ridged. 



III. Arca is so named from the broad, flat disc of the hinge 

 of the united valves, wliich has been likened to the 

 deck of a vessel. The animal of our typical species 

 spins a byssus ; it is called 



A, tetragona now, but more commonly known as A. Noa. 

 It is found in many localities, generally attached by its 

 byssus, in crevices of rocks, or in hollows of empty 

 shells. It is not so fine a shell as the foreign A, Nocb ; 

 it has a very broad area. 



A. lactea is small, white under the hairy epidermis, and with 

 a small area. 



A. raridentata is quite a minute shell, rather short, with 

 only three or four teeth on each side of the hinge. 



lY. Pectunculus has a thick, round, equilateral shell, with 

 the teeth arranged on each side of the hinge in a half- 

 moon; the muscular impressions are very strongly 



H 



