

1916.] NATURAL SCIENCES OF PHILADELPHIA. 441 



has not undertaken it, the writer hopes to make a descriptive classi- 

 fication of the animal communities of the sea beach in the Mount 

 Desert region. Provisionally he recognizes the following classifica- 

 tion of the Littoral Formation: 



/Drift Zone. 

 ILittorina rudis Zone 

 \ (Mean High Water). 



Solid Rock Association planus Zone 



uJbucus Zone.) 



/Mytilus Zone. 



(Transition Zone 



Littoral Formation^ \ (Mean Low Water). 



IRock Association. 

 'Rock Pool Association. 

 Boulder Association. 

 Shingle or Pebble Association. 

 Sand Beach Association. 

 Mud Flat Association. 



The mean rise and fall of the tide is ten vertical feet. 



Thais is found in the Solid Rock, Boulder and Rock Pool Asso- 

 ciations. In the two former it is confined to the Balanus, Fucus, and 

 Mytilus Zones, and particularly to the Balanus and Mytilus Zones, 

 as there the food is found. There, Thais is associated with Littorina 

 rudis, L. pallia ta, L. littorea, Balanus balanoides and Mytilus edidis. 

 When the tide is high the cunner (Tautogolabrus adspersus) and the 

 pollack (Pollachius virens) invade this formation to feed. 



Thais is not found in the Shingle, Sand or Mud Flat Associations. 

 Nor is it found alive in any association of the Bottom Formation, 

 unless we should classify the Rock Pool as a detached part of this 

 formation. 



It is not found in any stratum other than the upper surface of 

 rocks. It is found therefore neither on the weeds nor under the soil. 



Its habitat is very closely associated with its food. (See diagram, 

 fig. 1, illustrating the food relations of Thais to its organic environ- 

 ment.) 



The eggs are laid in little urn-shaped capsules in sheltered places. 

 They are more abundant in the Mytilus Zone in favored spots, such 

 as in cracks in the rocks or under masses of Fucus than in the Balanus 

 Zone. In each capsule about 400 eggs are laid. As cannibalism 

 is practised (Beard 1899, Carpenter 1857, Glaser 1905. Pelseneer 

 1904, Robert 1902) by the growing snail, a capsule containing snails 

 readv to hatch will hold but 8-14. 



