240 



W. C. ALLEE. 



of the open water most closely and the conditions in the inter- 

 tidal associations at the back of the flats are normally most 

 extreme, but in the middle regions the plant growth, mainly 



Table XVI. 



Flat Series. 

 Showing the average salinity, oxygen content, temperature and pH of different 

 animal associations of the flats as found in collections from the bottom, open 

 water excepted, at low tide, except for salinity where both low and mid tides are 

 averaged. Ordinal ranking is based upon variation from conditions prevailing in 

 open water. 



Associations. 



Salinity. 



Oxygen in 

 c.c. per L. 



Temperature. 



pH. 



Open 



Scoloplos fragilis 

 Phascolosoma . . . 

 Scoloplos aculus . 



Thyone 



My a 



Melampus 



5-17 



4-23 



6.15 



3. 



4-85 



3-72 



21. 1 



21.3; 

 23.3 



23-lj 

 22 I 



24 I 

 22.5 



IS I 8.0 



6 I 8.04 

 13 8.15 



17 7-95 

 8 7.9 



7 7-7 



7-3 7 



eel grass, affects conditions so that the relationship is irregular. 

 In the summation of ordinal rankings the effect of this confusion 

 is shown. 



The amount of range, Table XVII., also increases steadily as 

 one passes back on the flats only in the case of the salinity 

 measurements. The oxygen content and pH begin to increase 

 in observed range as soon as the eel grass is encountered in the 

 muddy sand association and reach their maximum range in the 

 tall eel grass of the Scoloplos acutus grounds. The water temper- 

 atures also increase in range but become greatest in the low water 

 of the marginal muck association. The surface in the older 

 associations undoubtedly becomes much warmer when exposed 

 to air and this must be particularly marked in the Melampus 

 association where the surface of the ground may be exposed to 

 the full glare of the afternoon sun with almost no protection 

 from the scant growth of marsh grasses. 



1 See note at bottom of Table XV. 



