BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 457 



(5) THE PLEUROCAPSA ASSOCIATION. 



Pleurocapsa juliginosa grows on rocks and stonework, forming a conspicuous dark 

 stain at high-water mark and in depressions wet by waves and spray. 



(6) THE ULVA, ENTEROMORPHA, AND MONOSTROMA ASSOCIATIONS. 



Rocks and stony beaches above low-water mark frequently exhibit striking growths 

 of species of Ulva, Enteromorpha, and Monostroma. Ulva Lactuca var. rigida is common 

 above low-water mark on rocks exposed to waves where it frequently forms dense 

 zones of growth. Enteromorpha intesiinalis is often abundant in quiet waters attached 

 to stones and shells and sometimes to woodwork of wharves between tide marks; it may 

 develop broad zones of growth in such situations. Enteromorpha lima is also found in 

 the same situations as Enteromorpha intestinalis and is sometimes mixed with it. Entero- 

 morpha minima is very common during the spring and summer in situations similar to 

 those of Enteromorpha intestinalis, but always growing near high-water mark. In the 

 spring Monostroma Grevillei is abundant on stones and larger algse a little above low- 

 water mark. 



These forms, together with certain species of Cladophora described in association 9, 

 make up the most characteristic associations of green algae in the littoral region. They 

 are generally responsible for the conspicuous green zones on wharves, rocks, and beaches 

 above low-water mark. 



(7) THE ULOTHRIX ASSOCIATIONS. 



Ulothrix flacca is not uncommon in the summer, forming large patches on stones 

 and woodwork of wharves above low-water mark ; it is sometimes epiphytic on Fucus. 

 Ulothrix implexa is also present in the spring on rocks above low water. 



(8) THE CH^TOMORPHA ASSOCIATIONS. 



Chaetomorpha Linum is common growing in wiry masses over sandy and muddy 

 bottoms. It was dredged by the Surs'ey as deep as 5 fathoms, but is generally found 

 in shallow water in the upper regions of both the cool- and warm-water sublittoral 

 formations (a and b). 



Chtrtomorpha melagonium is present in deeper water off exposed points, such as 

 Gay Head and Cuttyhunk (chart 228). This species was dredged in 4 to 9 fathoms and 

 is a characteristic member of what is here termed the cool-water sublittoral formation. 



(9) THE CLADOPHORA ASSOCIATIONS. 



Several species of Cladophora develop conspicuous associations in the upper level 

 of the sublittoral region. Cladophora alhida and C. alhida var. rejracta form in the sum- 

 mer patches on rocks. Cladophora arcta is very abundant in the spring on wharves and 

 harbor walls near low-water mark and below, and is one of the most characteristic of 

 the green algae at that season. C flexuosa is common in the summer on rocks, and C. 

 glauccscens, a delicate species, is also abundant at the same season on rocks and wharves 

 near low-water mark. C. gracilis grows luxuriantly during the summer in quiet sheltered 

 waters. C. lanosa is epiphytic on larger algae generally below low water; C lanosa var. 



