or TESTACEOUS MOLLUSCA. 
ARCA TRAPEZIA (t. 18, f. 40), Desh. Rev. Zool. 1839; Mag. Z. 
1840.—Reeve, I. Arc. Index; f.19, as A. Lopata (and in Z. P. 
1844). Hquivalve, solid, tumid, ovate-subtrapeziform, or oval-sub- 
trapeziform, very inequilateral, rufous-white under an umber-brown 
membranaceous epidermis, radiated with from 28 to 30 ribs, which 
on the posterior side are narrow elevated and nodosely crenated, but 
beyond the umbonal slope become smooth broader’ and more de- 
pressed ; dorsal edge angulated at each extremity; anterior end with 
a rounded-off angle at the ventral edge, which is convex, but arches 
obliquely upwards posteriorly ; umbones prominent; beaks remote ; 
ligamental area as in A. Diluvii; teeth numerous. 23... 33. 
Australia. 
ARcA RADIATA (t. 18, f. 45), Reeve, Z. P. 1844, p. 43; I. Are. 
Jf. 40. Inequivalve, oblong, inequilateral, ventricose, not very solid, 
angulated at both ends dorsally, white under a brown epidermis ; 
both valves with about 28 radiating ribs, that are square-cut, narrower 
than their indistinctly latticed intervals, and, except in front, 
nodosely crenated (indistinctly so in the smaller valve); ventral edge 
arched, rounded, and rising at both ends, very obliquely so behind ; 
no marked umbonal ridge; umbones prominent; beaks distant ; 
seated at the posterior third of the produced and lanceolate ligamental 
area, which is retuse and unsculptured. 1%...2. China. 
ARCA OBLIQUA (t. 18, f. 46), Reeve, Z. P. 1844, p. 48; I. Are. 
jf. 41. Equivalve, subovate, very oblique, very inequilateral, ven- 
trally produced in front, moderately thick, subventricose, wholly 
white under a light brown epidermis, that is setosely bristly between 
the ribs, which are about 36 in number, depressed but square-cut, at 
least twice as broad as their concentrically striated intervals, crenated 
near the front extremity, and bifid towards the hinder one; ventral 
edge moderately arched, rising peculiarly behind; anterior edge con- 
vex, forming an obtuse angle with the dorsal, rounded ventrally ; 
umbones approximated, seated at the third fifth of the rather narrow 
ligament ; inside quite white; teeth very numerous. 13. W. Africa. 
ARCA CHALCANTHUM (t. 19, f. 15), Reeve, Z. P. 1844, p. 44; 
I. Arc. f. 43. Inequivalve, subrhombic, thickish, more or less ven- 
tricose, whitish (stained inside at the edge with greenish blue), under 
a thin, green horny cuticle, that is partially coated again with a 
brown epidermis; rayed, in each valve, with about 28 narrow ribs, 
that are rather depressed, and, as well as their equally broad intervals, 
are smooth or nearly so; ventral edge obliquely arching upwards, 
behind; anterior side much the broader, perpendicularly sub- 
truncated; umbones prominent; ligament elongated; teeth very 
fine and numerous. 3...13. Philippines. In the larger valve the 
ribs are most obscurely subnodulous. , 
