31 



The lack of information regarding submerged populations 

 of Thais is readily explained, since such areas are inaccessible 

 to collectors, and are avoided by commercial fishermen opera- 

 ting trawls since they are destructive to their equipment. 

 However, during a number of years of exploratory activities 

 in the San Juan Islands a series of such reefs were located, 

 and samples of the snails secured. Many other reefs of a simi- 

 lar type appeared in the records presented by echo soundings, 

 but were not explored. 



The populations of T^^u found upon these submerged reefs 

 do not seem to differ in any marked respect, except in the 

 matter of size, from those occurring upon the adjacent inter- 

 tidal areas. Such populations may however, play an important 

 part as links in the migratory dispersal of the species. 



Predatory birds. 



It seems well agreed that predatory birds, including gulls, 

 crows, oyster-catchers and some species of ducks, represent 

 the most serious enemies of the snails. These birds on remov- 

 ing the animals from the rocks often carry them considerable 

 distances to find a suitable place to devour them, even drop- 

 ping them from a height to shatter them. Specimens escaping 

 from the birds might readily be added to the population of 

 another area. 



Human agencies. 



Although Dr. Dall failed to find remains of Thais in the kit- 

 chen-middens of the aborigines in Alaska, and concluded the 

 natives did not utilize these animals for food, the writer has 

 found an abundance of the fragmented Thais shells in the de- 

 posits left by the Indians in the region near Blaine, Washing- 

 ton. These people, in their migrations, may have unwittingly 

 caused some dispersal. 



Some colonies have been destroyed by engineering projects 

 and pollution, while others have been established through the 

 construction of docks, piers and breakwaters. 



Oysteimen and others, scowing material from one area to 

 another, may have modified some populations. 



