50 JOUKNAL OF THE NEW JEKSEY 



1. Dreissensia leucophfeta, Con. Shell crescent shaped, with the 

 posterior end broader and rounded; umbones terminal. Epi- 

 dermis rough, brownish ; interior not pearly. L. 13 ; H. 7 ; 

 W, 6. Animal with short siphons. — Maryland and south. On 

 oysters in brackish water. 



Family 5. ARCID/E, 



Shell regular, equivalve, with heavy epidermis ; ligament ex- 

 terior, occupying an area between the umbones ; hinge with a 

 long row of similar, comb-like teeth ; pallial line distinct and 

 without sinus; muscular impressions nearly equal. Animal 

 with open mantle, large, bent, deeply grooved foot, and very ob- 

 lique gills. 



1. Arca, L. 



Shell about equivalve, thick, subquadrate, closed (in our 

 species), with strong radiating ribs ; hinge straight, with numer- 

 ous teeth ; umbones anterior, separated by a lozenge shaped 

 space containing the external ligament. 



Animal with a long, pointed foot ; mantle furnished with ocelli; 

 animal usually attached by byssus ; palpi none ; gills long and 

 narrow. 



* Umbones ^ from anterior end 1. 



* Umbones J or more from anterior end 2, 3. 



1. Area pexata, Say. (Fig. 3.) Shell more rounded than the 

 next ; umbones turn obliquely forward and terminate over the 

 anterior end of hinge. Epidermis thick, shaggy, fibrous and 

 dark brown in color. L. 55 ; H. 42 ; W. 35 ; U.i — Mass. and 

 south. Not rare. 



2. Arca transversa, Say. Shell oblong, rhomboidal, with about 

 33 radiating ribs ; lower margin but little curved ; umbones situ- 

 ated near the anterior third of hinge as shown by the ligament 

 and the series of teeth. Epidermis chestnut brown. L. 37 ; 

 H. 25 ; W. 23 ; U.J.— Cape Cod and south. Not rare. 



3. Arca ponderosa, Say. Similar in shape to A. transversa but 

 the shell is heavier and the ribs are fewer in number and are 

 grooved along their centres ; the umbones end over the centre of 

 the series of teeth. L. 60; H. 48; W. 38; U. about J.— New 

 Jersev and south. 



Family 6. NUCULID/E. 



Shell oval to triangular, small, pearly within ; hinge with a 



