A RCA. 
CONCJHIFERA. 
J91 
Coral Rag, Mai ton, Yorkshire. 
7. Arca duplicata. —The Two-plaited Area, pi. LXXVII. 
lig. 1.9. 
A. duplicata . Sowerby, Y. pi. 474, fig. 1. 
Convex-ovate, transversely elongated, with double longitu¬ 
dinal ribs, which are furrowed along the middle; margin 
toothed ; marginal sinus obscure ; beaks approximate. 
London Clay, Hordwcll and Barton Cliff. 
8. Arca depress a.—T he Depressed Area, pi. LXXVII. 
% 2 k 
A. depressa . Sowerby, V. pi. 471, fig. 2. 
Compressed, transversely elongated; both sides rounded ; 
surface with distant elevated crenulated stria}, decussated by 
lines of growth, the striai upon the anterior side very wide-set, 
appearing like knotted threads; marginal sinus obscure. 
The Plastic Clay, Woolwich. 
9. Arca tumida.—T he Tumid Arca, pi. LXXVII. fig. 15. 
A. tumida . Sowerby, V. pi. 474, fig. 3. 
Very gibbosc, the depth of each valve nearly equalling its 
length, transversely elongated, with the anterior side acute ; 
marginal sinus short and deep; umbones remote, situate near 
the posterior side; surface with obsolete ribs. 
The Magnesian Limestone, Durham and Ilumbletou. 
10. Arca sub-acuta.— The Sub-acute Area, pi. LXXVII. 
fig. 33. 
A. sub-acuta. Sowerby, I. p. 95, pi. 44, upper figures. 
Gibbose, transversely oblong; its breadth exceeding its 
length ; hinge-line extending the whole length of the valves; 
surface longitudinally striated; marginal plaits rounded, and 
very deep, particularly at the anterior side; teeth sharp and 
numerous. 
The Chalk Marl, Hamscy. 
11. Arca carinata. —The Keeled Arca, pi. LXXVII. 
fig. 12. 
A. carinata. Sowerby, I. p. 90, pi. 45, lower figure. 
Very convex, parallelipe lal, its width twice its length ; 
anterior side flattened, separated by an acute angle, truncated 
at almost a right angle ; posterior side rounded; surface lon¬ 
gitudinally ribbed, every alternate one more prominent than 
the other. 
The LTpper Greensand, Hampshire. 
12. Arca rotund at a. —The Rounded Arca, pi. LXXVII. 
fig. 20. 
A. rotundata. Sowerby, Geo. Tr. 2d Ser. IV. p. 340, pi. 
17, fig. 8. 
Transverse; beaks small, quite contiguous; anterior side 
narrow ; posterior sido large and expanded, both extremities 
rounded; disk hollowed towards the base, where it is a little 
concave; surface with fino, radiating, longitudinal striae, and 
a few concentric fine lines of growth. 
The Greensand, Blackdown and Lyme Regis. 
13. Arca impolita.—T he Unpolished Arca. 
A. impolita. Sowerby, Geo. Tr. 2d Ser. V. p. 130, pi. 8, 
fig. 10. 
Transversely ovate ; very convex ; beaks small, hardly pro¬ 
truding; anterior side small, rounded; posterior side larger 
and rounded; binge and basal lines parallel to each other . 
surface with longitudinal lines of small punctures; substance 
of the shell thin. 
The London Clay, Hampstead. 
14. Arca nitens. —The Shining Arca. 
A. nitens. Sowerby, Geo. Trans. 2d Ser. V. p. 13G, pi. 8, 
fig- '*• 
Transversely ovate, convex ; anterior side small, somewhat 
rounded; posterior a little wedge-shaped; surface smooth and 
shining ; substance of the shell thin. 
In the London Clay, Hampstead. 
15. Arca papillosa. —The Pimpled Arca, pi. LXXVII. 
fig. 28. 
A. papillosa. Brown, Wernerian Mom. VIII. pi. 1, fig. 19. 
Transversely elongated ; beaks much produced, a little 
curved inwards, and remote from each other; hinge-line long 
and straight, with numerous small teetli ; ligamental area 
broad and elongated; anterior side shortest and rounded ; 
acute above ; posterior side obliquely truncated ; a ridge ema¬ 
nating from the umbo terminates on the lower angle of the 
truncations; basal and hinge lines parallel; surface with nume¬ 
rous divergent, longitudinal, papillose ribs ; and a few strong 
transverse lines of growth. 
The Pleistocene Marine Formation, Portrush, Ireland. 
16. Arca costata. —The Ribbed Area, pi. LXXVII. f. 32. 
A. costata. Brown, Mem. Man. Geo. Soc. I. p. GG, pi. G, 
figs. 34, 35. 
Transversely oblong-ovate; anterior side very short and 
acute; the posterior lengthened and sub-truncated; beaks 
small and slightly produced ; surface with three or four pro¬ 
minent ribs emanating from the beaks and terminating on the 
posterior side, with many nearly obsolete concentric Vriukles. 
The New Red Sandstone, Newton, Manchester. 
17. Arca jemula. —Emulating Area, pi. LXXVII, fig. 3G. 
A. amiula. Phillips, Geo. York, 1. pi. 3, fig. 29. 
Transversely elongated, oblique; bjaks large, obtuse, and 
approximating; disk slightly hollowed; base a little concave 
in the centre ; surface with divergent, longitudinal striae, in¬ 
terrupted by the transverse lines of growth, which are pretty 
strong, and following the concave direction of the basal line. 
The Coral Rag, Maltou, Yorkshire. 
18. Arca lactea. —The Milk-white Arca. 
A. lactea . Brown, Illust. Couch. Gt. Brit, and Ird. p. 86, 
p*. XXXIII. fig. G. 
Transversely oblong, and slightly oblique; beaks obtuse, 
remote; posterior side a little angulated; somewhat open at 
the base for the passage of a byssus ; longitudinally ribbed, 
and crossed by numerous lines of growth, producing a decus¬ 
sated aspect; with the interstices punctured ; margin plain. 
The Pleistocene Marine Formation, in the Forth. 
19. Arca elongata. —The Elongated Arca, pi. LXXVII. 
fig. 34. 
Transversely elongated ; oblique ; beaks very obtuse ; liga¬ 
mental area of medium width ; anterior side angular above 
and rounded below ; posterior sub-truncated obliquely, rather 
acute above, and rounded below ; hinge-line and base nearly 
parallel, the latter a little concave in the centre ; surface with 
fine divergent, longitudinal striae; crossed by remote, rather 
regular lines of growth. 
In the Greensand, Blackdown. 
20. Arca cylindrica. —The Cylindrical Arca. 
A. cylindrica. Purtlock, Geo. Rep. p. 428, pi. 34, fig. 9. 
