II. — A Study of the family Pectimd^e, with a Revision of 

 THE Genera and Subgenera. (Six plates.) By A. E. 

 Vekrill. 



The classification of the Pectinidse must, for the present at least, 

 be based mainly upon the characters of the shells, for the soft parts 

 have been carefully studied only in a few of the very numerous spe- 

 cies. There is good reason to believe that in some cases good gen- 

 eric characters may be found in the structure of the foot, but this 

 organ is so contractile that alcoholic specimens give but a poor idea 

 of its form in life. The palpi and gills are known to afford import- 

 ant characters in some species, but they have been studied in only a 

 few genera, (See Plates xx, xxi.) But the relations of the shell to 

 the soft parts are so very intimate that the form and structure of the 

 shell may be taken as an expression of the modifications of some of 

 the important internal parts, and therefore must give valid evidence 

 of generic modifications. 



In this family the use of shell-characters for generic and subgen- 

 eric groups has this great advantage that it will enable us to classify 

 the vast number of fossil species, which are far more numerous than 

 the living ones, and to compare those of successive geological periods, 

 group by group, with each other and with modern groups. In this 

 way Ave ma}'^ be better able to trace the lines of evolution. Until 

 the minor modifications of structure, such as charactei'ize subgenera 

 and " sections " of genera in this and other bivalve families, are duly 

 considered, no great progress can be made in the study of their evo- 

 lution. 



It is very essential that students of Mollusca should become im- 

 pressed with the idea that even the slightest modification of the 

 foi'm or structures of the shell, if persistent, has its meaning, and 

 some distinct value in progressive evolution, whether we maybe able 

 to discover it or not. Even the color is often of protective value. 

 For instance, the reddish and brown colors of Chlaniys Islandica 

 matches the colors of the red nullipore that covers the stony bottoms 

 where it is usually found, and C. irradians has generally a gray 

 color, similar to that of the sandy bottoms, which it mostly fre- 

 quents. Instances of protective coloration in the shells of gastropods 

 are very numerous. 



