TRIBE ARCACEA. 161 



A. Emarginata. Sow. Z. P. 1833. Oblong, subcylindrical, 

 inequivalve, white, with radiating ribs : posteriorly short with the 

 ribs narrower, and wrinkled ; anteriorly elongated, with the ribs 

 broader and smooth : anterior dorsal margin emarginated, liga- 

 mental area narrow ; epidermis brown and bristly in the grooves. 

 |- . . If. — C. America. 



A. Tuberculosa, So7v. Z. P. 1833. Oval, turgid, oblique, 

 subauriculated, with numerous radiating ribs armed especially pos- 

 teriorly with scattered tubercles : umbones close, ligamental area 

 narrow: anterior side the longer. 2J^.,2|. — S. America. 



A. Cardiformis. Sow. Z. P. 1833. Inequivalve, suboval, 

 rounded posteriorly, on the anterior side ventrally angulated : ven- 

 tricose, whitish with radiating ribs which are smooth except on the 

 posterior side where they are wrinkled, the interstices on the larger 

 valve very narrow : lateral edge sloping : ligamental area small 

 and subequal. l^f^ . . 2. — California. — A.^j)ect of the co7nmon. 

 Cockle. 



A. Multicostata. Sow. Z. P. 1833. Ovate-rhombic, pos- 

 terior side angulated dorsally, and rounded ventrally ; anterior side 

 angulated dorsally, lateral edge sloping ; a rounded keel running 

 from the umbo to the base of the anterior margin : white under 

 a brown almost satin-like epidermis, with numerous radiating mi- 

 nutely decussated rounded ribs, which posteriorly are subgranular : 

 the grooves rounded, the ligamental area broadish. 2|- . . 2^. — 

 C. America. 



A. Nodosa. Wood. S. t. 2. f 8. Equilateral, tumid, ob- 

 liquely subrhombic, white ; hinge margin angulated at both ends, 

 very obliquely so anteriorly ; rayed w^ith about tAventy-six flat ribs 

 which angulated at their sides are rather broader than the simple 

 interstices, and are decussated, except on the anterior slope, with 

 transverse straight raised lines : ventral edge arcuated : ligamental 

 area rhombic broad and short. 2. — S. America ? 



A. Transversa. Sai/. Journ. A. N. S. Phil. 2. p. 269.— 

 Gould. Mas. p. 97. Oblong, rhomboidal, with from thirty-two 

 to thirty-five ribs as wide as their interstices, apices separated by 

 a long narrow area (on which one or two angulated lines diverge 

 from the apex) and situated at one-third the distance from the 

 posterior end of the hinge margin which is angulated at each end ; 

 of the anterior edge the superior half rectilinear, posterior edge 

 rounded, ventral edge nearly straight, -ij . . l^. — U. States.'^ 



» See also the Complanata of Ch. f. .544, 5. (D. p. 228.— 

 W. t. 9. f. 8.) and his Plicata f. 2008 {D. p. 22/. and f.— W. 

 t. 9. /. 6.) 



