242 W. C. ALLEE. 



of these rankings arranges these associations in order of their 

 age but when combined with the rankings from average condi- 

 tions shown in the preceding table, the marginal muck {Thy one) 

 association is placed slightly earlier in the series than it belongs. 

 Further data would probably adjust this arrangement. 



At the surface of this series, the range of oxygen, pH and 

 temperature is greatest in the Scoloplos acutus region. The mat 

 of eel grass just at the surface at low tide allows a surface layer 

 of very warm water to be found in the bright sunlight where it 

 is supersaturated with oxygen and has a correspondingly high 

 pH. The L. litorea, S. lucice, Bugula, C. convexa, B. alternatum, 

 Molgula, and Botrylhis, which occupy this region must be very 

 resistant to these extreme conditions. Under the most pro- 

 nounced conditions few animals capable of moving are found at 

 the surface. 



The differences on the flats level off at high tide. The tide 

 appears to come to the back of the flats over the surface of the 

 more stagnant water which has remained behind during low tide, 

 bringing lower temperature, higher salinity, lower oxygen content 

 and lower pH. Thus two collections from the back of the flats 

 in 4.5 feet of water on a flowing tide showed a specific gravity of 

 1.018 at the bottom and 1.022 at the surface. Obviously such 

 conditions do not prevail long and by diffusion the gradients 

 disappear. In the long eel grass (S. acutus association), the 

 vertical gradient at low tide in one foot of water was found to be 

 from 3.37 c.c. oxygen per liter to 10.39 a t the surface. Three 

 hours later in the same place in three feet of water of the new 

 tide the gradient was from 7.03 at the bottom to 8.3 at the 

 surface. Similarly a pH gradient of from 7.7 at the bottom and 

 8.7 at the surface became one of 8.0 to 8.1. 



The condition of the water over the bottom of the flats at 

 high tide tends to become uniform throughout all the associa- 

 tions. Thus at NW. Gutter Flats, collections made under 

 typical high tide conditions showed a range of 0.1 in pH over the 

 flats while at low tide the range at similar stations was over 1.2. 



V. Discussion. 



Sumner (1908) in discussing the study of the distribution of 

 bottom living animals in the Woods Hole region considered 



