SUMMARY 



1. This paper deals with the complex group of local races 

 of the marine gastropod Thais lamgllosa (Gmelin) inhabiting 

 the west coast of North America from Santa Cruz, California^ 

 to the Aleutian Islands in Alaska. 



2. An account is given of the taxonomic history of the 

 species, which illustrates the difficult nomenclatorial problem 

 presented by extremely variable polytypic forms. 



3. The biology of the species is discussed, including its life- 

 history, food habits, means of dispersal, distributional barriers, 

 injurious organisms, and ecological adjustments. 



4. The geologic history of the forms of Thais inhabiting the 

 Pacific is reviewed, and the probable relationship of these to 

 the Atlantic species Thais lapillus is suggested. 



5. The existing distribution of the immense number of local 

 rar*»9 of Thais lamellosa^ differing not only among themselves, 

 but also exhibiting great variation among the individuals com- 

 prising the local populations, is interpreted as due to a com- 

 plex set of factors, including the genetic instability of the 

 ancestral stock, geographic isolation acting over a long period 

 of time throughout the entire range of the species, microgeo- 

 graphic isolation involving local topographic and hydrographic 

 situations, and nutritive conditions in highly localized niches. 



6. All of the diverse phenotypic forms are interpreted as due 

 to the inbreeding of genetically segregated groups, which de- 

 termines the form of the shell, its sculpture, including both 

 spiral and axial bands, and its coloration. Each of these is re- 

 garded as determined by separate and independently acting 

 genetic factors. 



7. The only characteristic regarded as determined primarily 

 by environmental conditions is that of size, which is governed 

 by nutrition. Where food is abundant, and the feeding process 

 is not interrupted by tidal exposure, the animals grow rapidly 

 and attain a greater size at maturity. This tends to modify 

 the sculptural pattern, since the several elements are given ^ 

 greater spread. 



