II 



Since the appearance of Tryon*s monogaph of the Purpur- 

 inae in 1 880 a number of important papers have been published 

 deahng with the taxonomy, Hfe history, ecology and genetics 

 of this group, which have tended to modify and extend our 

 earlier interpretations. 



The photographs of Thais shells accompanying this paper 

 are for the most part approxim.ately natural size. The proce- 

 dure followed has been to arrange a group of shells upon a 

 surface measuring 5x7 inches, which were then photographed 

 upon a film of the same size. Where there has been any de- 

 parture from this scale it will be indicated in the material 

 accompanying the several plates. Groups of shells with pre- 

 vailingly pale coloration have been photographed upon a black 

 background, otherwise a white surface has been utilized. 



In selecting groups of shells for illustration the writer has 

 attempted to show the general character of the inividuals com- 

 prising the specific populations, and also the tendency of the 

 individuals to vary among themselves to form a local cline, 

 either as regards sculpture or coloration, or both of these. In 

 some cases extraneous elements appear in a population, since 

 the degree of isolation varies with distance, tidal currents 

 and the driving power of storms. 



It is of course needless to say that our knowledge of the lo- 

 cal races of Thais is far from complete. The great area of our 

 North Pacific coast, extending from Puget Sound to Bering 

 Sea, includes a vast number of islands, rocky promontories 

 and reefs, very few of which have been explored by zoologists. 



ACKNOWLEDGMENTS 



As has already been indicated the writer is greatly indebted 

 to Miss Bertha Challis for her fine cooperation in organizing 

 the Thais collection and helping, many years ago, to lay the 

 foundation for the present study. 



The writer is greatly indebted to Mr. Walter J. Eyerdam 

 of Seattle, Washington, for his unfailing assistance in securing 

 local populations from areas that otherwise wjuld have been 

 inaccessible. Eyerdam has collected extensively in Alaska, 



