372 TESTACEOUS MOLLUSCA. 



up behind; ligamental area short and very narrow. £...£. Medi* 

 terranean. 



Arca scabra (t. 18, f. 37), Poll, t. 25, /. 22, teste Phil. 1, p. 58 ; 

 2, p. 42. Obliquely subrhomboid, very inequilateral, somewhat com- 

 pressed, except at the umbones, reddish brown, with close concentric 

 sulci, the raised intervals of which are nodosely decussated, forming, 

 as it were, interrupted series of radiating lyrae ; beyond the umbonal 

 ridge, winch is angular near the sharp and obliquely inclined beaks, 

 the surface assumes a granular aspect. Ventral edge simple, slightly 

 hiant near the incurved middle, arcuated at each end, obliquely rising 

 posteriorly; ligamental area rather narrow. £...y. Mediterranean. 



Arca illota (t. 18, f. 41). — Byssoarca i. Sow. Z. P. 1833. — 

 A. i. Reeve, I. Arc./. 78. Equivalve, inequilateral, not solid, ventri- 

 cose, rotundately subcuneiform in front, rounded and much shorter 

 behind, wholly white beneath a thin fuscous foliaceous epidermis ; 

 radiated with fine but projecting costellse, that are closer and sub- 

 nodulous at the extremities, and largest on the rounded umbonal 

 ridge; dorsal edge short; ventral edge gaping; anterior margin 

 arcuated ; umbones swollen and prominent ; beaks diverging from 

 each other near the enlarged end of the short ligamental area. 

 f...l^. Cent. America. There are some rather obscure and distant 

 concentric raised wrinkles. 



Arca virescens (t. 18, f. 47), Reeve, Z. P. 1844,^?. 124; I. Arc. 

 f. 97. Equivalve, elongated, very inequilateral, not ventricose, 

 greenish white under a thin brown epidermis, which is bristly in 

 front ; radiated with simple raised striae, which are distant anteriorly, 

 yet close elsewhere ; ventral edge gaping ; front extremity produced, 

 rotundately subtruncated, subemarginated dorsally ; ligament rather 

 narrow ; inside quite white ; teeth indistinct in the middle. £ . . . 1£. 

 Philippines. 



Arca tenella (t. 18, f. 48), Reeve, Z. P. 1844, p. 124 ; I. Arc. 

 f. 91. Date-shaped, equivalve, oblique, very inequilateral, thin, ven- 

 tricose, white or brownish white, clothed with a slight epidermis of 

 scattered hairs ; densely, subequally and finely grooved throughout, 

 the radiating sculpture subgeniculated by rather indistinct concentric 

 wrinkles ; ventral gape slight ; sides rounded ventrally ; umbones 

 prominent; beaks obtuse, incurved, and subdivergent posteriorly;* 

 ligamental area lanceolately acuminated in front ; dilated but indis- 

 tinct behind; inside wholly whitish. £...l£. Philippines. The 

 sculpture is so fine that the surf ace feels smooth. The site of the um- 

 bonal ridge is rounded. 



Arca maculata (t. 18, f. 49). — Byssoarca m. Sow. Z.P. 1833. — 

 A. m. Reeve, I. Arc. f. 71. Subrhombic, biangulated in front, 

 rounded behind, inequilateral, thickish, ventricose, whitish, stained 



