ARCA. ARK . 
8 
white, rounded at the posterior end, elongated and a little 
angular at the other, covered with an olive-brown sldn, 
and beset with a marginal beard of strong brown hair-like 
hj T ssus, with numerous nearly equidistant fiat longitudinal 
ridges which are covered with fine short thickset brown 
hair, and crossed with numerous fine lines giving it a reti¬ 
culate appearance ; the hollows at the anterior and poste¬ 
rior slopes are also covered with bristles of a dark umber 
color: beaks nearer one end, pointed and close together; 
inside rather glossy, with pale rays, and clouded with pale 
chesnut; the margin finely notched : teeth in a straight 
l ine, rather small, but increasing in size as they recede from 
the centre : length half an inch 5 breadth rather more than 
an inch. 
It differs from A. barbata, in having the margin notched, 
and in being a little angular on the anterior side. 
A perfect specimen was found alive, adhering to an 
oyster, from Killinchy in Lough Strangford, Ireland, by 
Dr. Macgee of Belfast. 
4. Area Nucleus. Silvery Ark. Fig. 1, 2. 
Da Costa , pi. 15. f. 6 . right hand fig.— Linn. Tram. vi. 
pi. 18. f. 3 to 6 — Donovan , pi. 63— Dorset Cat. pi. 12. f.G, 
right-hand fig. 
Shell somewhat triangular, slightly convex, producedon 
one side, covered with a smooth olive-green skin, under 
which it is white with generally some yellowish rays from 
the beak to the margin, and mostly marked with fiesh-co- 
lor or blueish transverse hands and a few coarse ridges, 
finely and regularly striate longitudinally: beaks smalt; 
turning towards the shorter side, beneath which is an oval 
depression; inside bright silvery : teeth in a curved line, 
about twenty on the elongated side, and half the number on 
the other, separated by a triangular cavity: length half aa 
inch or more; breadth not so much. 
On most sandy shores, v. v. 
B. TVith the margin very entire. 5, to 11. 
5. Area minima. Least Ark. 
Shell somewhat globular, a little produced on one side, 
transparent, glossy, smooth or so minutely striate longitu¬ 
dinally 
