50 TSE 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 K'ucula (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 (Area pexata, 

 PI. 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 



