42 A. £J. Yerrill — Study of the family Pectinidce. 



In the PectinidjB we can appreciate the value of some of the mod- 

 ifications of the shell, on purely mechanical principles. Others are 

 known to be correlated with the habits of the species. Thus the 

 strong radial ribs or corrugations, found on species living in shallow 

 water, serve to give their shells great strength, while the interlock- 

 ings of the ends of the ribs, either in the form of marginal points or 

 scallops, serve to keep the valves in exact apposition when closed, 

 and therefore compensate for the absence of cardinal teeth. Such 

 corrugations and marginal projections are generally lacking in the 

 deep-sea species that are not exposed to the action of the waves. 

 The special, internal, radial ribs of Amusinm and allied genera also 

 serve to greatly strengthen the thin, smooth shells of this group and 

 enable the valves to resist the action of the powerful adductor mus- 

 cle in the act of sudden contraction for the purpose of swimming. 

 In this group, the very compressed, round and polished shell indi- 

 cates an adaptation to very active swimming habits, for such a shell 

 gives the least friction in the singular manner in which these bivalves 

 swim. Our large native scallop ( C. Glintonms or Magellanicus) has 

 a similar form and is remarkable for its swimming powers, even 

 when of large size. 



It seems difficult to explain satisfactorily Avhy Amiisiuni, and 

 forms like C. Clintonius^ should have a simple, thin margin, without 

 interlocking points or scallops, and often with the shell incapable of 

 . closing tightly. It is possible that many such simple-edged and gap- 

 ing shells have descended from those with radial ribs and interlock- 

 ing scallops, for many of the ancient mesozoic fossil species are thus 

 defended. It is true that most (but not all) of the species that have 

 no radial corrugations, and no marginal scallops, are from deep 

 water, where they are not exposed to the rough action of waves and 

 currents. Still they are, even there, exposed to the attacks of 

 various crabs and fishes, against which strong interlocking valves 

 would be an obvious advantage. It seems probable that the increased 

 lightness of the shell, by facilitating rapid swimming, may more 

 than compensate for the loss of the power of passive resistance. 

 This might well be the case whenever their principal enemies are 

 sluggish animals, like drilling gastropods and voracious starfishes, lor 

 actively swimming Pectens could easily escape from such enemies, 

 while their heavier and more sendentary relatives, especially those 

 attached either directly or by a strong and persistent byssus, might 

 be unable to escape. 



Experience in the extensive cultivation of oysters, on the American 



