50 THE SHELL-FISH OF THE COAST. 
where. The scallop, unlike the oyster, is a fairly 
active creature, moving about with rapidity by 
means of its finger-shaped foot. In the young 
condition the animal swims freely through, and on 
the surface of, the water, propulsion being effected 
mainly by the sudden opening and closing of the 
valves. A scallop placed high and dry will at slow 
intervals open its shell, and then suddenly close it 
with a peculiar thud, a performance that is re- — 
peated until the animal is wellnigh dry. The part 
of the scallop that is used for food is the thick 
white muscle which holds the two valves of the 
shell together. 
A few words about arks and their allies. These 
can be immediately recognized by the large num- 
ber of teeth on the hinge-line, by means of which 
the two valves are kept firmly interlocked. In the 
arks proper (PI. 3, Figs. 12, 15, 17) these minute 
comb-like teeth are arranged in a continuous linear 
series, but in the pearl-lined Nucula (Figs. 10, 16) 
and in Yoldia (Fig. 11) the series is interrupted, 
and broken in outline. In the arks the shell, when 
not badly worn, is coated with a heavy epidermis, 
usually of a dark brown color. Many of the species 
spin a stout byssus, which serves as an anchor- 
line of attachment to the surfaces of stones, 
rocks, etc. The shells of three species are found 
on our coast, each well defined by peculiarities in 
their outline. The rounded form (Arca pevata, 
Pl. 3, Fig. 15), which differs from most arks in 
having a minimum of space between the two valves, 
is further distinguished by the possession of red 
