APPENDIX* 373 



with chocolate on the subuodulous aud concave umbonal slope, closely 

 fimbriated by gradate concentric ridges, that are decussated by still 

 more dense radiating lyrae ; umbonal ridge angular ; beaks pointed, 

 seated near the end of the tawny-brown ligament, which is elongated 

 and broadish ; inside white, stained with chocolate in front, and on 

 the hinge-margin; teeth numerous. li. Pacific. The ventral 

 edge is usually incurved and slightly gaping. 



Akca Reeve an a (t. 18, f. 51, as A. squamosa, var.), D'Orb. Am. 

 for A. Helblingii, Reeve, Ic. (not Brug.J. Equivalve, subrhombic- 

 oval, compressed, moderately thick, subinequilateral, white, with a 

 rich brown shining somew T hat scaly epidermis, roughened throughout 

 by very numerous radiating costellse, that are crowded and not much 

 raised in the middle of the shell, where they are geniculated by 

 the ridge-like wrinkles of increase, and are much more prominent, 

 larger and more distant at the sides, where they are closely 

 and nodosely crenated. Ventral edge gaping. Beaks seated at, or 

 before, the second third of the length of the produced, narrow T and 

 sunken ligamental area, wdnch is set with groove-like lines that 

 diverge in very obtuse angles from the acute and prominent apices. 

 Extremities with obtuse angles dorsally, rounded ventrally. Inside 

 pure white ; teeth very small in the middle, abruptly larger at the 

 sides. l£...2-£. W. America. 



Arca decussata (t. 19, f. 16). — Byssoarca d. Sow. Z. P. 1833, 

 p. 18. — A. d. Reeve, I. Car. f. 81. Equivalve, inequilateral, thick, 

 subventricose, oblong or oval-oblong, bluntly angulated dorsally, 

 rounded ventrally, white, with an epidermis of dirty brown pointed 

 scales ; most closely radiated with very narrow costellae, that are 

 very closely and minutely decussated, the anterior riblets nodosely 

 crenated, their intervals narrow ; ventral edge gaping, arcuated ante- 

 riorly ; ligament produced, narrow, evenly lanceolate ; beaks rather 

 acute and projecting ; inside wholly white. 1^...2. Pacific Isles. 



** (Area proper). Ventral edge regularly notched or 



toothed. 



Arca Diluvii (t. 18, f. 38). — A. antiquata, Phil, (not Lin.) 1, 

 p. 59, t. 5,/. 2. — A. d. Phil. 2, p. 43. Obliquely suboval, equivalve, 

 very inequilateral, solid, tumid, white, radiated with nearly 30 rounded 

 ribs, which, as well as their nearly equal deep intervals, are closely 

 striated concentrically ; dorsal edge angulated at each extremity ; 

 anterior side with a rounded-off angle at the notched and arcuated 

 ventral edge, which rises obliquely behind ; umbones prominent ; 

 beaks remote; ligamental area tawny -brown, oval-lanceolate, with 

 numerous close grooves that form very obtuse angles : teeth numerous. 

 1 * ... 1 -£%. Mediterranean. 



