BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 457 
(5) THE PLEUROCAPSA ASSOCIATION. 
Pleurocapsa fuliginosa 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 intestinalis 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 linza is also found in 
the same situations as Enteromor pha intestinalis and is sometimes mixed withit. 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 Greville: is abundant on stones and larger alge 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 alge 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 CHATOMORPHA ASSOCIATIONS. 
Chetomorpha Linum is common growing in wiry masses over sandy and muddy 
bottoms. It was dredged by the Survey 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). 
Chetomorpha 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 albida and C. albida var. refracta 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 alge at that season. C. flexuosa is common in the summer on rocks, and C. 
glaucescens, a delicate species, is also abundant at the same season on rocks and wharves 
neat low-water mark. C. gracilis grows luxuriantly during the summer in quiet sheltered 
waters. C. /anosa is epiphytic on larger alge generally below low water; C. /anosa var. 
