PECTEN. 49 



and the less convex valve uppermost. But as the " beak" 

 of a shell is posterior, and that of a bird is 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 Ostreida. The shell 

 is of a more regular and symmetrical shape, and its 

 hind margin is expanded on each side into the ears or 

 wing-like 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 conspi- 

 cuous 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 Pectinida 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. PI. II. f. 1. 



BODY oval : mantle fringed with ocelli or eye-like tubercles. 

 SHELL more or less inequivalve : ligament internal : muscu- 

 lar scar nearly central. 



The name of this genus is nearly as ancient as that 

 of Ostrea. It is very expressive, the shell usually 

 having ribs which are arranged like the teeth of a 

 lady's comb. Sometimes it resembles the expanded 



* A comb. 



D 



