254 DIMYARIA,—ARCADZ. 
occasionally adhere by means of their fibrous 
epidermis. 
The species of this genus are chiefly distributed 
through the seas of the tropics, where they are 
NOAH’S ARK. 
very numerous. We have three or four on our 
own shores, the largest of which is commonly 
known as the Ark of Noah, though the species 
properly so named (A7ca Now) is a native of the 
Mediterranean. Ours is the Arca tetragona of 
conchologists. The valves are deep, four-sided, 
and strongly angular, covered with fine radiating 
lines, of a warm reddish-brown hue, becoming 
paler and sometimes white on the front part. The 
valves sometimes attain a breadth of two inches, 
with a length of about three-fourths of an inch. 
It is found commonly all round the British coasts, 
most abundantly in the south-west, and in the ex- 
treme north. ‘It is taken in crevices of rocks, in 
chinks of old shells, in the interior of dead shells, 
and sometimes quite free. When found in con- 
