TRIBE ARCACEA. 159 



A. Cayennensis. Lam. 3G. 01)liquely oval, pectiiiiform, red- 

 dish yellow, with at least thirty radiating uuarmed ribs, which 

 are angular but flattened on the surface ; anterior side broad and 

 obhquely rounded, furrows visible within. — Variety. Smaller, 

 obhquely heart-shaped, and the ribs slightly crenated. li. — 

 Guiana. 



A. BisULCATA. 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. li. — 

 Guiana if Brazil. 



A. Incongrua. 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 . . 2^. — U. States. Very like 

 Rhombea, but thejigure in the Encyclopedle dijfers. 



A. PoNDEROSA. Say. Jour. A. N. S. Phil. 2. p. 267- Some- 

 what obUque, 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. 2^ . . 2f .— U. States. 



A. Labiosa. 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 I'ibs which (especially on the larger 

 valve) are posteriorly granular, anteriorly smooth : ligamental area 

 narrow. W^ • . 1 o%- — Peru. 



A. Pexata. Say. Jour. A. N. S. Phil. 2. p. 268.- Gould. 

 Mas. p. 95. /. 60.— A. Americana. W. S. t. 2./. 1. Trans- 

 versely ovate subquadrate, very inequilateral, heavy, with from 

 thirty-two to thirty-six close set ribs, which ai-e 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. l^. — St. Dominyo. 



