TRIBE ARCACEA. WAL 
Jj. 62. Oblong, very smooth and shining, subequilateral, cuticle 
light green occasionally with darker zones, and two or three lighter 
radiations : anterior dorsal edge rectilinear nearly to the tip, which is 
a little recurved pointed and not truncated: ventral edge scarcely 
at all incurved: inside bluish white, subnacreous, twenty-two 
posterior, eighteen anterior teeth. -%,..1-9;.— United States. 
* * & Elliptic. 
N. Marearitacea.* Lam. 6.—Philippi. p. 64.¢t. 5. f. 8. 
—Sow. G. f. 7. (good.)\—Arca M. Brug. Dic. p. 109.— 
Arca Nucrevus. Lin. 1143..—D. p. 244.—Brooks. Intro. 
t. 3. f. 36.—Da Cost. t. 15. f. 6.—Petiv. t. 17. f. 9.—Gualt. 
t. 88. R.—W. t. 10. f. 42.  Ovate subtriangular, moderately 
convex, nearly smooth, some extremely fine radiating striz 
being indistinctly visible near the crenated ventral edge ; cuticle 
olive-yellow ; umbones prominent ; ventral edge rounded ; anterior 
side much the longer, its dorsal edge moderately sloping ; posterior 
edge all but direct (but not abrupt), convex: beaks acute and re- 
curved: lunule defined by a shallow groove, inside pearly, hinge 
with numerous straight sharp pectinated teeth. _An. Var? (N. 
Por? Phil. p. 63. t. 5. f. 8.) Larger, more oblique and 
triangular, olivaceous, with ridges of growth, posterior edge abrupt, 
anterior more slopmg.—Lngland, France, Sc. 
N. Decussata. Som. Conch. Il. f. 18. Somewhat tri- 
angular, white, decussately striated; posterior side shorter, its 
slope rather elevated in the middle; anterior side much longer, 
oblique ; ventral margin rounded, its edge crenated. 3..17.— 
Guinea. 
N. Rucutosa. Sow. Conch. Il. f. 19.  Ovate triangular, 
with a greenish epidermis covered with rugulose- strie; angles 
obtuse: posterior slope with a central elevation; anterior slope 
somewhat rounded ; ventral margin very finely crenulated. ¢.. 4. 
N. Niripa.* Sow. Conch. Il. f. 20. Ovate, subtriangular, 
smooth, shining, white with an olive coloured epidermis: anterior 
slope somewhat rounded, posterior nearly straight, with a central 
elevation. 2..4. 
N. Osriaua. Lam. 5.—Son. Cone. Il. f. 21. Obliquely 
ovate, subelliptical, thin, pellucid, olive, smoothish ; posterior side 
rounded, rather long in proportion to those of this division, its 
edge convex: anterior side rounded, its dorsal edge much arcuated, 
the margin very entire. }..3.— Valparaiso, South Seas. 
N. Tenurs.* Turt. B. p. 177.—F. p. 402.— Gould. Mas. 
p. 105. f. 64.—Arca T. Mont. Sup. p. 56. t. 29. fi 1.— 
Pen. 4. p. 215.—D. p. 246.—Turt. D. p. 11.—W. t. 10. 
Jf. 45. Ovate-trapezoidal, thin, smooth, without radiating lines, 
glossy grass green; anterior dorsal line scarcely sloping, nearly 
bole 
