86 
MOLLUSCA. 
1. Arc A No^, pi. XXXIII, f. 1, 2, 3. 
Area Nocb, First Ed., pi. 25, f. 1, 2, 3; Linne, Syst. Nat., 
p. 1140, No. 169; Lamarck, VI, pt. 1st, p. 37 ; Montagu, p. 
139, pi. 4, f. 3, 3 ; Donovan, V, pi. 158, f. 1, 2 ; Turton, Biv., 
p. 166; Fleming, p. 397- 
Shell rhomboidal, cymbiform ; the smaller end of each valve 
turning inwards to meet each other ; umbones situate nearest 
to one side, separated by a pretty wide cardinal area, which is 
smooth and glossy; apex prominent, slightly inflected; hinge 
parallel, with numerous teeth, those in the centre perpendicular, 
and oblique towards the sides; external surface with numerous, 
longitudinal, divergent striae, which emanate at the apex, and 
terminate on the margins, those on the narrower side wider 
and stronger, assuming nearly the aspect of ribs, the whole 
crossed by fine, transverse striae, producing an elegant reti- 
culated appearance ; colour rufous-brown ; inside white, or 
slightly tinged with purplish-brown, with the margins slightly 
crenated. 
Found on Milton Sands, south coast of Devon ; at Land’s 
End, Cornwall ; and also at Guernsey. 
2. Arca fusca, pi. XXXIII, f. 4, 5. 
Arcafusca, First Ed., pi. 25, f. 4, 5; Lamarck, VI, pt. 1st, 
p. 39; Donovan, V, pi. 158, f. 3, 4 ; Montagu, Sup., p. 51 ; 
Turton, Biv., p. 167 ; Fleming, p. 397. 
Shell cymbiform, transversely elongated; umbones prominent, 
rounded, obtuse, and somewhat remote, being separated by a 
pretty broad, flat, cardinal area, and situated nearer one end ; 
whole outer surface fuscous-brown, covered with fine, divergent, 
longitudinal striae, which are decussated by transverse, sharp 
striae ; posterior side somewhat truncated ; inside glossy, and 
white ; margin with very fine crenulations. 
Found at Cornwall and south coast of Devon. Lady Jardine 
possesses a specimen which was found near Weymouth. 
This shell differs from A. Noce in being longer in proportion 
to its breadth, in the striae being finer, and the colour more of 
a yellowish-brown. 
3. Arca lactea, pi. XXXIII, f. 6. 
Arca lactea, First Ed., pi. 25, f. 6 ; Lamarck, VI, pt. 1st, p. 
40; Montagu, p. 138; Donovan, IV, pi. 135; Fleming, p. 398; 
Arca perforans, Turton, Biv., p. I69j ph 13, f. 23. 
Shell subrhornboidal, rounded at both extremities ; umbones 
not quite central, blunt, and placed somewhat remote ; cardinal 
area smooth ; hinge parallel, with numerous teeth ; outer surface 
covered with a pilous, pale yellowish-brown epidermis, beneath 
which the shell is milk-white, with numerous, close-set, regular, 
longitudinal striae, the interstices between the striae finely punc- 
tured, producing the appearance of being reticulated, when 
viewed through a lens, crossed by a few concentric wrinkles, or 
lines of growth; inside white, with a plain margin. Length half 
an inch ; breadth three-quarters. 
The sides in the young shells are for the most part oblique, 
and subtruncated, and aissume a more rounded form as the shell 
advances in growth; some specimens have also transverse striae, 
which, however, is but of rare occurrence. 
Found plentifully on the Devonshire and Cornwall coasts. 
4. Arca barbata, pi. XXXIII, f. 7- 
Arca barbata, First Ed., pi. 25, f. 7 ; Brown, Wernerian 
Mem., II, p. 512, pi. 24, f. 3; Lamarck, VI, pt. 1st, p. 59; 
Fleming, p. 398 ; Arca reticulata, Turton, Biv., p. 168. 
[Cardiacea. 
Shell transversely elongated, strong, white, very finely reti- 
culated ; covered with a reddish-brown epidermis ; from the 
umbones to the margin diverge a number of nearly equidistant, 
flat ridges, covered with very fine, short, and thick-set brown 
hairs ; all round the margin it is beset with a byssus of rather 
long, stifiF bristle-like hairs; in the hollows at the anterior and 
posterior sides, it is thickly covered with bristles of a very dark 
umber-brown, rather small at the umbo, and increase in size as 
they diverge from it on both sides, and oblique in opposite 
directions ; inside moderately glossy, growing dimmer as it 
approaches the umbones of the shell, white, with pale glossy 
rays, and clouded with pale chestnut; margin finely crenelated. 
Breadth nearly double its length. 
Discovered in Lough Strangford, County of Down, Ireland, 
by Dr. Macgee, of Belfast. 
5. Arca tetragona, pi. XXXIII, f. 20, 21. 
Arca tetragona, Forbes, Mai. Mon., p. 41 ; Poli, p. XXV, 
pi. 25, f. 13? 
Shell transversely oblong, obliquely quadrangular, very tu- 
mid ; an oblique rib takes it rise at the umbo, and terminates 
on the anterior side ; umbones but slighily produced, and very 
remote, separated by a very large, flat, nearly lozenge, cardinal 
area, from the sinistral side of which it suddenly sinks, forming 
an acute angle, by the meeting of the arcuated basal line ; car- 
dinal area with a large lozenge impression, subquadrangularly 
striate, the lines of the one valve meeting those of the other ; 
whole surface with longitudinal and transverse strim ; colour 
brownish-white; covered with a thin, pilous epidermis, and 
fringed round the margins of the valves, which are longer at 
the beak ; basal margin provided with a large, central, oblong, 
lozenge hiatus, for the passage of the byssus; inside white, 
tinged with purple at one side ; hinge line straight, with many 
teeth, which are numerous posteally and few anteally, the mar- 
gin with fine crenulations. 
Found in the Calves on the West coast of Ireland, where it 
burrows in hard clay and limestone rocks ; and dredged by 
Professor Forbes oflF the coast of Ballaugh, Isle of Man. I 
obtained it from Mr. Richardson, with one of the specimens, 
embedded in limestone. 
Family III — Cardiacea. 
Primary teeth irregular both in form and situation, and in 
general accompanied by one or two lateral teeth. 
Genus 7 — Isocardia Lamarck. 
Shell equi valve, heart-shaped, ventricose; beaks very distant, 
divergent, and involute ; hinge with two primary compressed 
teeth in each valve, the one next the apex inflected under the 
umbo, and with one elongated, lateral tooth, situate imme- 
diately before the ligament, which is external, and divided into 
two ligaments at its posterior extremity, both of which are 
divergent to the point of the beak in each valve ; both valves 
provided with two lateral, ramote, muscular impressions, the 
linear impression of the mantle is entire, and extending from 
one muscular impression to the other. 
1. Isocardia Cor, pi. XXX, f. 9, and pi. XXX,* f. 5. 
Isocardia Cor, First Ed., pi. 23, 24; Lamarck, VI, pt. 1st, 
p. 31 ; Turton, Biv., p. 193, pi. 14; Fleming, p. 419; CTiama 
