1916.] NATURAL SCIENCES OF PHILADELPHIA. 117 



(2) Light — pure white, together with intense and dilute chestnut 

 with white lip. The outer layer of the shell of these is white. 



(3) Dark — chestnut and purple in all their combinations. 



(4) Yellow — pure yellow. 



(5) Striped— both narrow and wide stripe in all their combinations 

 and color. 



(6) Imbricated — in all the combinations of patterns and color- 

 In the tables that follow the number of snails found in each group 



is stated in per cent, of all the snails in each collection from a given 

 station. 



The Relation of the Varieties to the Environment. 



The base for this study was Greenings Island, an island locat< d 

 between Northeast and Southwest Harbors in the mouth of Somes 

 Sound. It is triangular in shape, with a perimeter of about two and 

 a quarter miles. The main mass of the island is composed of till, 

 which at one time formed the terminal moraine of the continental 

 glacier before it retired from Somes Sound. This till is protected 

 from rapid wave erosion by outcrops of reddish granite at the east 

 and west end of the island. This granite is cut in several places by 

 broad basalt dykes which run in a general north and south direction. 

 Under wave erosion the till between the outcrops of solid rocks has 

 given material to form a boulder-strewn beach and in favorable 

 places the finer products have given rise to pebble and sandy beaches. 

 On the west or more sheltered side of the island between the granite 

 outcrops are a number of muddy coves. 



On this small island we have all the principal Littoral associations 

 present. The distribution of these are found in fig. 4. This figure 

 is a diagram of the shore of Greenings Island, with the vertical scale 

 very much exaggerated. It has been made as though the shore were 

 cut at the northwestern point and folded out so that this north- 

 western point appears at each end of the diagram. The character 

 of the shore is indicated. The island as a whole is protected from 

 heavy surf by outlying islands, but the eastern end is exposed to the 

 ocean swell from both the south and cast, and at times of storm- i- 

 subject to quite a surf. The northeast shore and south shore are 

 relatively unprotected at the east end, but become more protected 

 toward the west. On the diagram the per cent, of pure white shells 

 are placed in the locality in which the collection was made. 



An inspection of this figure will show that the most unprotected 

 portion has the lighest colored snails, while the portion of the island 



