A. E. Verrill — Study of the family PectinidcB. 43 



coast, and in other parts of the world, shows that starfishes and the 

 drilling gastropods are by far the most destructive enemies of those 

 bivalves. On our shores vast numbers of drilled oyster shells can be 

 found almost everywhere, but drilled shells of our common scallop 

 [Chlamys irradians), which is an active swimmer, are comparatively 

 rare. Therefore it is probable that the gradual loss of radial ribs 

 and corrugations, or their failure to develop in certain genera, is due 

 to natural selection, in consequence of the advantage gained by the 

 lighter shells for swimming purposes, in escaping from these slug- 

 gish enemies. 



Concentric ribs and undulations, found on some very thin shells, 

 serve to stiffen and strengthen the shell against transverse strains, 

 bi;t they tend, also, to facilitate the tight closing of the valves at the 

 simple and thin margins, for they permit a certain degree of flexi- 

 bility of the thin shell, parallel with the margin. This kind of clo- 

 sure is very obvious in Propeamusium and Cydopecten, which 

 include many small, thin, deep-water forms. In some cases the clo- 

 sure is still farther perfected by a flattened or bevelled margin. 



Most members of the family, if not all, form a byssus while young, 

 for attachment, but they release themselves very easily and swim 

 actively away. Many large .and thick species seem to lose the habit 

 entirely at maturity, and to rest unattached upon the bottom. But 

 some small and delicate species, although capable of active swim- 

 ming, appear to live attached much of the time through life. This 

 is the case with Camptonectes or PaUiolum vitrea and some allied 

 deep-water species, which attach themselves to the branches of gor- 

 gonians, corals, and hydroids, and thus gain protection from their 

 enemies. The presence of a byssus is, however, consistent with the 

 most active swimming powers. (See remarks under Cydopecten^ 

 p. 71). 



The extreme inequality of the valves in typical Peden {^iJanira) 

 is a singular character, of ancient origin, for it was fully devel- 

 oped in many mesozoic species, closely allied to modern forms. It is 

 the more strange, because, in most of the other groups having 

 unequal valvf s, it is the under, or right, valve that is the flattest, but 

 in true Pedens the right valve is strongly convex, while the left or 

 upper valve is flat, or even concave externally, and usually shuts inside 

 the margin of the lower valve like a lid. Both valves are thick and 

 strongly ribbed. Probably this shape is advantageous when the 

 shell is resting upon the bottom, with the lower valve partly buried 

 in the sand and gravel around it, but not attached by a byssus, for 



