372 TESTACEOUS MOLLUSCA. 
up behind; ligamental area short and very narrow. }...4. Medi- 
terranean. 
ARCA scaBRA (t. 18, f. 37), Poli, t. 25, f. 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 lyre; beyond the umbonal 
ridge, which is angular near the sharp na 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. 4...3. Mediterranean. 
ARcA ILLoTA (t. 18, f. 41)—Byssoarca 1. Sow. Z. P. 1863.— 
A... Reeve, I. Arc. f. 78. Equivalve, inequilateral, not solid, ventri- 
cose, rotundately subcuneiform in frout, rounded and much shorter 
behind, wholly white beneath a thin fuscous foliaceous epidermis ; 
radiated with fine but projecting costelle, 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 lgamental area. 
3...13. Cent. America. There are some rather obscure and distant 
concentric raised wrinkles. 
ARCA VIRESCENS (t. 18, f. 47), Reeve, Z. P. 1844, p. 124; I. Are. 
f. 97. Equivalve, elongated, very imequilateral, not ventricose, 
greenish white under a thin brown epidermis, which is bristly in 
front; radiated with simple raised strive, 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. 3... 14. 
Philippines. 
ARCA TENELLA (t. 18, f. 48), Reeve, Z. P. 1844, p. 124; I. Are. 
f. 91. Date-shaped, equivalve, oblique, very inequilateral, thin, ven- 
tricose, white or brownish white, clothed with a shght 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. 3...13. Philippines. The 
sculpture is so fine that the surface feels smooth. The site of the wn- 
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 
