THE CHARACTER AND DISTRIBUTION OF THE 

 LOCAL RACES OR MORPHS OF Thais Umellosa 



As has been indicated, the writer is of the opinion that the 

 great diversity and localized character of the immense num- 

 ber of populations of Thais lamellosa inhabiting the Pacific 

 Coast of North America, is due primarily to genetic segrega- 

 tion in an extremely unstable hereditary constitution and the 

 effects of geographic and microgeographic isolation. 



The limited powers of dispersal in this species, together 

 with the existence of innumerable barriers in the way of deep 

 water channels and areas of uninhabitable terrain, has lent 

 itself to a process of speciation on a grand scale. The ability 

 oi lamellosa to inhabit waters subject to periods of extremely 

 low salinity, has enabled it to form localized races in the estu- 

 aries of rivers, often separated by many miles of inhospitable 

 terrain, as is illustrated by the populations found in Willapa 

 Bay, Coos Bay and elsewhere. 



The relationship of Thais lamellosa to the other species of 

 this subgenus inhabiting the same coastal areas is evidently 

 quite close, but although populations of lamellosa are com- 

 monly found associated with those of the other forms, no evi- 

 dence of cross-breeding has been observed. The two groups 

 also differ ecologically, owing to the greater ability oi lamellosa 

 to endure conditions of low salinity. 



Although it would be premature, in the absence of any defi- 

 nitive experimental data, to speculate on the probable relation- 

 ship between the superficial phenotypic appearances and the 

 underlying genetic organization, it is obvious the heritage of 

 this species must be governed by several sets of independently 

 acting units, determining respectively, sculpture, spire form, 

 coloration and size. We find for instance, the same type and 

 range of coloration in populations exhibiting the greatest 

 diversity of form and sculpture, indicating complete inde- 

 pendence . From the fact that many populations are without 

 pigmentation, while in others various types of banding are 



