30 



Where areas of shore forming suitable habitats for the snails 

 are separated by stretches consisting of mud, sand or loose 

 gravel, the migration of the animals is inhibited, with resulting 

 isolation. The extent of such inhibiting stretches may vary 

 from a few hundred yards to many miles, as in the case of the 

 great sandy beaches on the main ocean frontage. One of the 

 commonest type of such isolating barriers is the crescentic 

 gravelly beaches such as are frequent in the northwestern area 

 extending from Puget Sound to Alaska. 



III. Islands and reefs. 



A glance at a map of the northern section of our Pacific 

 Coast, discloses at once the existence ot a vast number of is- 

 lands of various sizes and types. Many of these, as for instance 

 Vancouver Island, are sufficiently large and diversified as to 

 shoreline to support a considerable number of local popula- 

 tions, and as a result the total number of these must be very 

 great . 



IV. Channels. 



The width and depth of channels separating bodies of land 

 bears directly upon the problem of dispersal. The force and 

 direction of the tidal currents presumably determine the pri- 

 mary paths of migration in the establishment of new colonies, 

 or the modification of those already in existence, and has 

 probably been a decisive factor in determining the existing 

 distributional pattern of the populations. 



V. Submerged reefs. 



Although Thais is regarded as primarily an intertidal form 

 it has long been known that Thais lapillus can maintain an 

 existence at considerable depths where suitable conditions ex- 

 ist, and the same situation holds true for Thais lamellosa. 



Very little seems to be known regarding the extent and eco- 

 logical conditions in these submerged populations, beyond 

 the fact that in such areas "giant" forms are apt to prevail. 



