47 



distributional pattern of the species, when presumably it was 

 segmented for vast periods of time as the glaciers advanced 

 and retreated during the Pleistocene, and the shifts in the level 

 of the ocean profoundly affected the character of the intertidal 

 zone. With the final retreat of the ice sheet the coastline was 

 gradually exposed, with its islands and rocky headlands. Into 

 this biological vacuum the intertidal fauna, including the sev- 

 eral forms of 7'^aw, gradually penetrated. Among the types of 

 lamellosa that had maintained itself to the southward of Cape 

 Flattery was a form in which the sculptural pattern had be- 

 come greatly reduced. Spreading through the Strait of Juan 

 de Fuca this form established colonies on both sides of the 

 strait, and then invaded areas in the San Juan Islands and the 

 Canadian islands to the northward. A further extension car- 

 ried it to the northern section of Puget Sound and into areas 

 bordering on Rosario Strait. The distribution of this type of 

 population is illustrated by the morphs shown in Plates xxvii, 

 :(Xix-xxxii. Many additional related populations are at hand. 



A second invasion of the vacated area would seem to have 

 been by a type of population which the writer is inclined to 

 regard as more distinctly southern in origin. It is marked by a 

 relatively blunt spire and a moderate development of the ver- 

 tical lamellae. This type developed colonies in the Strait of 

 Juande Fuca, and extended with many modifications into the 

 San Juan Islands, and into the Canadian islands to the north- 

 ward, but became the most characteristic form throughout 

 the extensive shoreline of Puget Sound and Hood Canal. Thi« 

 type of population is indicated by the morphs shown in Plates 

 xxxiv-xxxviii. In a group of bays in the western section of 

 Puget Sound some populations with an extremely imbricated 

 shell have been found (Plates xlix,l), which would seem to 

 be an extreme modification of this type. 



A third group of populations would appear to have invaded 

 the ice- freed area from the north. It is characterized by the 

 presence of an elongated spire and extensive imbrication. The 

 main center for the distribution of this group is in Southeast- 

 ern Alaska, where many striking forms are present, as may 



