l6o OYSTERS, AND ALL ABOUT THEM. 



that which is known as systole and diastole, and by 

 repeatedly taking in and expelling a quantity of water, it 

 flits or jerks itself along for a considerable distance, 

 although not in a straight line, flapping the valves of its 

 shell inwards like the wings of a bird in full flight. The 

 ventral margins are in front, the beaks are behind, and the 

 less convex valve uppermost. But as the "beak" of a 

 shell is posterior, and that of a bird anterior, their relative 

 position is reversed, although the motion of each is nearly 

 the same. Even Pecten pusio in its younger state ; and 

 before it is permanently fixed, is free and can swim about 

 like its congeners. In other respects also this family 

 differs from the Osfreidce. The shell is of a more regular 

 and symmetrical shape, and its hind margin is expanded 

 on each side into the ears or winglike processes above 

 noticed. Its structure is less compact its composition, 

 according to Mr. Sorby, being " arragonite," while that of 

 the oyster is " calcite." The impression of the great 

 adductor muscle is placed more on one side than in the 

 oyster, where it is almost central. The muscle by which 

 the front edge of the mantle is attached to the shell leaves 

 a conspicuous scar on the inside of each valve. The 

 cartilage and ligament advance with the growth of the 

 animal, in the same way as in the oyster, but more slowly. 

 All the British Pectinidce are ribbed or striate lengthwise. 

 Pecten similis, which is commonly smooth, is hardly an 

 exception, for it sometimes has distinct ridges diverging 

 from the beak to the margin of the shell. 



Genus I. PECTEN. Pliny. PL 2. f. i. 



BODY oval : mantle fringed with ocelli or eye-like 

 tubercles. 



