154 AMERICAN MARINE CONCHOLOGY. 



yond a regular curve at the middle ; basal margin crenate within ; 



pale livid, with or without obscure rays. 



Length 12.5 mill. 



New Jersey. 



This very distinct species appears to be extremely local in dis- 

 tribution ; it does not occur north of Long Island Sound, nor has 

 it been detected south of Delaware Bay. 



2. D. variabilis, Say. Figs. 378, 379. 



Journ. Philad. Acad., ii. 305, 1822. 



Shell triangular; anterior margin obliquely truncated, cordate, 



suture a little convex; posterior hinge margin nearly straight; 



base a little prominent beyond a regular curve near the middle. 



Valves striated longitudinally with scarcely visible parallel 



impressed lines; basal edge crenate. White or bluish with raj-s 



of bluish-purple. 



Length 22, height 12 mill. 



Beaufort, N. C, southwards. 



Genus ABBA, Leach. 

 Lam. Anim. s. Vert. 1818. 



1. A. iEQUALis, Say. Figs. 380, 381. 



(Amphidesma.) Journ. Philad. Acad., ii. 307. 1822. 



Shell orbicular, slightly oblique, polished, white, with very 

 minute and numerous concentric wrinkles near the margin, which 

 are obsolete on the disk and umbo ; lateral teeth none ; primary 

 teeth two in the left valve and one in the other; interior ligament 

 cavity subfusiform, as long as the exterior ligament. 



Length 10 mill. 



New Haven, Conn., North Carolina, southwards. 



Genus SEMELE, Schumacher. 

 Essai Nov. Gen., 165. 1817. 



1. S. orbiculata, Sa}'. Fig. 382. 



(Amphidesma.) Journ. Philad. Acad., ii. 317. 1822. 



Shell orbicular, somewhat compressed; beaks nearty central and 



a little prominent ; valves slightly wrinkled concentrically ; hinge 



with two lamellar teeth. White. 



Length and height 27 mill. 



North Carolina, southwards. 



2. S. radiata, Say. Fig. 383. ' 



(Amphidesma.) Journ. Philad. Acad., v. 220. 1826, 



Transversely oval orbicular, a little compressed. Apex nearly 



