ARCACEA. | 
Section I1.— Shells suborbicular. 
5. NucuiA MarGaritacea, pl. XXXII], f. 12. 
Nucula margaritacea, First Ed., pl. 25, f. 12; Lamarck, VI, 
pt. Ist, p.59; Forbes, p. 43; Nucula nucleus, Turton, Biv., p. 
176, pl. 13, f.4; Arca nucleus, Pennant, IV, p. 217; Montagu, 
p- 141; Donovan, I], pl. 63. 
Shell thick, inflated, obliquely ovate, slightly subtrigonal; 
umbones rather prominent, placed nearest the posterior side ; 
beneath them a cordiform depression; colour olive, of various 
shades in different individuals, and very glossy; with numerous, 
concentric, nearly obsolete wrinkles, crossed by numerous, fine, 
interrupted strie; inside of a glossy, highly nacred silvery white ; 
hinge angular, with numerous, upright, sharp teeth, extending 
considerably above the hinge lines; margin minutely crenulated. 
Length half an inch; breadth somewhat more. 
Found on the Devonshire, Welsh, and Yorkshire coasts; also 
in many parts of Scotland; Dublin Bay, Portmarnock, Bray, 
and the Downshire coast, Ireland. 
6. NucuLa ARGENTEA, pl. XXXIII, f. 14, 15. 
Nucula argentea, First Ed., pl. 25, f. 14, 15. 
Shell obliquely ovate, very glossy, and smooth; colour oliva- 
ceous; umbones placed much to the posterior side, with a 
cordiform depression beneath; inside glossy, silvery white ; 
hinge with twelve rather strong, regular, upright teeth; margins 
with very fine crenulations. Length and breadth not quite a 
tenth of an inch. 
One valve only of this shell was discovered at Dunbar, by 
General Bingham, and is in his cabinet. It has all the appear- 
ance of the fry of NV. margaritacea, but differs from it in the 
following particulars, viz.: in its subovate form; having no lon- 
gitudinal striz; hinge line gradually curved, and not triangular, 
as is NV. margaritacea ; ligament pit in a line with the teeth, 
and does not project beyond them; and the teeth are less 
numerous. 
7. Nucuua TENUuIs, pl. XXXIII, f. 13. 
Nucula tenuis, First Ed., pl. 25, f. 13; Turton, Biv., p. 1773 
Arca tenuis, Montagu, Sup., p. 56, pl. 29, f. 1. 
Shell somewhat ovate, and conyex, smooth, and very glossy ; 
of a pale olivaceous colour, with a few concentric, nearly obso- 
lete wrinkles; umbones rather small, prominent, reflected, and 
placed nearest the posterior side, with a slight cordiform depres- 
sion under them; inside smooth, white, and somewhat nacred ; 
margin thin, entire, and divested of crenulations; hinge with 
about sixteen elevated, imbricated teeth, ten on the one side 
of the cartilage pit, and six on the other ; cartilage socket pro- 
jecting inwards. Length not a quarter of an inch; breadth a 
quarter. 
Found in the Frith of Forth; the estuary of the Clyde; and 
the coast of Downshire, Ireland. 
GeENus 5.—PeEctTuncuLus.—Lamarck. 
Shell orbicular, subequilateral, with the valves close; umbones 
near to each other, and separated by a narrow facet, or area; 
hinge semicircular; teeth numerous, arcuated, oblique, serrated, 
placed in two rows, one on each side of the umbones, and sepa- 
rated by a small triangular disk in each valve, which contains 
the ligament, those of the opposite valves alternately lock 
between each other, and becoming nearly obsolete towards the 
Y 
MOLLUSCA. 85 
umbones; two lateral, strongly marked, distant, muscular im- 
pressions, which are united by an interrupted pallial impression ; 
ligament external. 
1. PrEctTuNcuLus PiLosus, pl. XXXIII, f. 10, 11. 
Pectunculus pilosus, First Ed., pl. 25, f. 10, 11; Lamarck, 
VI, pt. Ist, p. 49; Turton, Biv., p. 172, pl. 12, f. 2; Fleming, 
p- 400; Forbes, p. 42; Pectunculus undatus, Turton, Biv., p- 
173, pl. 12, f. 3,43 Arca pilosa, Montagu, p. 136; Ib., Sup., 
p- 53; Maton and Rackett, Linn. Tr., VIII, p. 94, pl. 3, f. 4. 
Shell orbicular, strong; surface pale reddish-brown, or white; 
with zigzag, angular, stripes of dark chestnut, or reddish-brown, 
frequently of a flesh-colour in young shells, with fine, longitu- 
dinal striw, and transverse, irregular wrinkles; above which the 
surface is covered with a dark brown, pilous epidermis, which 
is generally extremely thin, or none, near the umbones, and 
thicker and longer towards the margins and base of the valves ; 
umbones large, central, and rounded, inclining slightly towards 
each other, and separated by a polygonal, dark blackish-brown 
area; inside white, or slightly tinged with purple near one side; 
margin strongly crenated. Diameter two inches and a half, 
sometimes more. 
In the young condition the surface is distinctly decussated, 
but the transverse stria become obsolete as they advance in 
growth. The P. decussatus of Turton is merely the young of 
this species. 
Found on the south coast of Devonshire, Weymouth, and 
the Kentish coast. Professor Forbes dredged it of a large size 
in the Irish Channel, off the Manx coast, where it is plentiful, 
He particularises the coast off Douglas Head, as being a locality 
where it is abundant. 
2. PrEctuncoLus Grycimeris, pl. XXXIII, f. 8, 9. 
Pectunculus Glycimeris, First Ed., pl. 25, f. 8, 9; Turton, 
Biy., p. 171, pl. 12, f. 1; Arca Glycimeris, Maton and Rackett, 
Linn. Tr. VIII, p. 93, pl. 3, f. 3. 
Shell orbicular, convex, thick, strong; umbones distant, 
round, and pointing towards each other; hinge line with a poly- 
gonal area of a deep brown colour, but not striate; whole sur- 
face of a pale yellowish-white, with nearly obsolete, wide-set, 
longitudinal, and transverse strie, irregularly and remotely 
spotted with yellowish-brown; covered with a dark, subpilous 
epidermis; margin strongly crenated, and acute at the edge; 
cardinal teeth transverse, about ten in number. Diameter two 
inches and one-eighth, and upwards. 
Distinguished from the P. pilosus by being much more ven- 
tricose, in the teeth being fewer in number, and with the epi- 
dermis less pilous than in that shell. 
Found on the Western coasts, Cornwall, and British Chan- 
nel, but is very rare. 
Genus 6.—Arca.—Linneus. 
Shell transverse, or subequivalve, inequilateral, trapeziform, 
or subquadrate ; slightly ventricose ; some species greatly yen- 
tricose ; generally angular at both ends of the hinge line, much 
rounded in some species; umbones small, remote, separated by 
the area, to which the external ligament is affixed; hinge line 
rectilinear; teeth numerous, small, serrated, close-set, alter- 
nately inserted in the opposite valves; two lateral and distant 
muscular impressions ; ligament external. 
