452 PROCEEDINGS OF THE ACADEMY OF [June, 



was only .47%. The largest number found was but 3|% at any one 

 station and that was on the rocks of a muddy harbor. The station 

 in Somes Harbor farthest away from the ocean had but .6% imbricata. 



That there are other geographical factors is shown by the rarity 

 of imbricata in the Somes Sound region. Very few are found in 

 exposed situations on the Cranberry Islands and on Seal Harbor 

 Head, situations exposed to the full effort of the surf. Except for 

 a colony at the Southwest Harbor steam-boat wharf, in the whole 

 Somes Sound region they are indeed rare. Out of 2,765 shells 

 collected on Greenings Island but three were imbricated and these 

 three were found at one station on the muddy west side of the island. 



However, in general, we can say that the imbricated forms are 

 found in greatest abundance in exposed situations, as Duck Island 

 and Long Island, and also in the most sheltered positions. 



Most of the varieties found in the Atlantic are also found in the 

 forms inhabiting the north Pacific Ocean. Dall 2 reports that the 

 lamellated forms of Thais in that region are found best developed 

 in sheltered harbors. The Atlantic lamellated forms we have seen 

 may occur in sheltered as well as in exposed situations. 



As Thais is found in both the Balanus and the Mytilus Zones, is 

 there any difference between collections made close together but 

 in different zones? The an.swer to this question is summed up in 

 the following table (Table V) compiled from collections made but a 

 few feet apart, but in different zones. 



Table V. 



Imbri- 

 Locality. Zone. Sta. Lt. Wt. Yel. St. cata. 



Greenings Id., 



eastern point Balanus. 2 44.5 21.3 ? 



Mytilus. 50 37.5 22.5 14.4 



,Steam-boat wharf, 



S. W. H Balanus. 37 29.5 19. 17.5 1.5 19.0 



Mytilus. 47 14.5 6.4 4.1 37 19.2 



Stanley House 



wharf, S. W. H Balanus. 35 25.3 11 2.2 4.0 



Mytilus. 34 21.4 4.8 5.3 



(1) There are more light forms in the white Balanus Zone than 

 in the dark Mytilus Zone. 



(2) There are fewer yellow forms in the dark Mytilus Zone. 



(3) Striped forms not conclusive. 



(4) Imbricated forms not affected. 



2 Proc. U. S. Nat. Mus. t vol. 49, pp. 557-572. 



