TRIBE ARCACEA. 159 
A. CayeNnensts. Lam. 36. Obliquely oval, pectiniform, red- 
dish yellow, with at least thirty radiating unarmed ribs, which 
are angular but flattened on the surface; anterior side broad and 
obliquely rounded, furrows visible within.—Variety. Smaller, 
obliquely heart-shaped, and the ribs slightly crenated. 11.— 
Guiana. 
A. Bisutcata. Lam. 37. Transversely oblong, anteriorly an- 
gulated, longitudinally grooved, the ribs transversely striated and 
alternately smaller, yellowish or reddish white: inner margin 
obscurely crenulated.—Variety. With crenulated ribs. 14.— 
Guiana § Brazil. 
A. Inconerva. Say. Jour. A. N. S. Phil. 2. p. 268. 
Somewhat rhomboidal with from twenty-six to twenty-eight close 
set ribs crossed by elevated obtuse equal and equidistant lines 
except on ten ribs of the disc of the left valve; apices opposite the 
middle of the hinge, distant with a narrow lanceolate area between 
them : dorsal edge angulated at each end, posterior edge rounded, 
ventral edge rounded, that of the right valve extending a little 
beyond the regular curve in the middle : anterior margin cordate, 
anterior edge nearly straight. 2..21.—U. States. Very like 
Rhombea, but the figure in the Encyclopedie differs. 
A.Ponpnrrosa. Say. Jour. A. N.S. Phil. 2. p.267. Some- 
what oblique, very thick and ponderous, with from twenty-five to 
twenty-eight ribs, each marked by an impressed line and as wide as 
the interstices; umbones, very prominent, apices remote and op- 
posite the middle of the hinge; ligamental area with longitudinal lines 
as prominent as their corresponding teeth ; anterior margin cordate, 
flattened, distinguished from the disc by an abrupt angular ridge, 
posterior edge rounded very short, ventral nearly straight or in- 
curved in the middle, 21..23.—U. States. 
A. Lapiosa, Sow. Z P. 1833. Inequivalve, short, of a 
rounded square shape; posteriorly shorter, angulated dorsally and 
rounded ventrally, anteriorly subangulated ; whitish under a thin 
brown epidermis, with radiating ribs which (especially on the larger 
valve) are posteriorly granular, anteriorly smooth : ligamental area 
narrow. 1,3,..1,%.—Peru. 
A. Prexata. Say. Jour. A. N. S. Phil. 2. p. 268.— Gould. 
Mas. p. 95. f.60.—A. Americana. W. 8S. t.2. f.1. Trans- 
versely ovate subquadrate, very inequilateral, heavy, with from 
thirty-two to thirty-six close set ribs, which are broader than their 
interstices ; a shaggy brown epidermis; beaks prominent and very 
oblique, apices approximated : posteriorly obtuse and (save when 
young) destitute of a dorsal angle; anteriorly obtuse, with an angle 

fulvous ; anterior side produced, subacute and granular; beaks 
approximated, margin indistinctly crenated. 13.—S8t. Domingo. 
