128 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



modern pectens are becoming longer. This must, however, remain an 

 hypothesis until measurements have been made on the British fossil 

 Pecten opercularis. 



2. Symmetry in Pecten. 



Pecten belongs to a group (Lamellibranchiata) which is characterized, 

 on the whole, by a high degree of bilateral symmetry; i.e., the right 

 and the left valves tend to be equal in all their axes. Some remarkable 

 exceptions to this rule, however, occur ; as in the case of the oyster, the 

 family Chamidce, and the now extinct family of Hippuritidce. In all 

 such cases the mollusc rests on one valve, either the right or the left, 

 and it is usually stated that " the fixed valve, whether right or left, is 

 always deep, and the free valve flat." * Pecten rests invariably upon 

 the right valve, and this is (Tryon, 1882, p. 288) in most species the 

 more convex. This is, indeed, true for Pecten maximus and even Pec- 

 ten irradians, but it is not true for Pecten opercularis, despite its close 

 relationship to the latter species ; for in Pecten opercidaris the lower valve 

 is the flatter. f The ratio of the breadth of the upper valve to the lower 

 valve is, however, not constant for shells of all sizes. The facts are 

 shown in Table II. 



TABLE II. 



Ratio of Breadth of Upper Valve divided by Breadth of Lower Valve 

 for Shells of all Sizes from the Irish Sea. 



This table shows that in the smaller shells, of G2 mm. breadth, the 

 ratio is 1.35, that it rises from this to 1.55 at about 72 mm., and then 

 falls to 1.36 in shells whose lower valve has a breadth of 117 mm. The 

 meaning of this series is quite obscure. Some would see in it evidence 

 of selection tending in later life to weed out the more unsymmetrical 

 individuals ; others would draw the conclusion that the ancestors of 



* Cooke (1895), 271 ; Tryon (1882), 53. 

 t This fact was noted by Lamarck (1819). 



