54 THE RELATION OF PLANTS TO TIDE-LEVELS 
Microcoleus chthonoplastes occurs in rope-like bundles more or less abun- 
dantly scattered through the mats of Rhizoclonium, Lyngbya, etc., just men- 
tioned. It also occurs in nearly pure blackish-green gelatinous patches on the 
otherwise bare mud among Spartina stems between the 4-foot and 6.5-foot 
levels. It is found also in the next higher zone, being there mixed with other 
Schizophycee. 
Microcoleus tenerrimus occurs in the tangled mats above mentioned, but is 
less abundant than the last species and has not been found in pure growths. 
Oscillatoria lumosa is frequently a sparse constituent of the matted coverings 
of the pebbles on the south shore of the Spit at 6 feet and upward (e. g., at 500 
east). ‘T'wo other species also occur here, one of them distinguished from 
O. limosa by its constantly straight tips and the other by its smaller diameter, 
which is less than half of that of O. lumosa. 
It is probable that other species of Oscillatoria and of other Schizophycee 
which occur at higher levels will be found in the present belt, but the lack of 
time made it impossible to determine accurately the distribution of any but the 
more abundant species of these minute plants. 
Rwwularia atra forms shiny black pebbly patches, on otherwise bare, firm mud, 
between the 5 and 6 foot levels. It is most abundant on the vertical or overhang- 
ing bank formed by the caving off of the Marsh on the east side of the Creek 
at 100 to 200 south by 775 east. Here, in the shade of the overhanging grasses 
of the bank, many square decimeters of surface are covered more or less con- 
tinuously by patches from half to several centimeters in diameter. These 
patches are distinguishable at once from those of the Anabena mentioned above 
by their pebbly surface, greater thickness (1 to 2 mm.), and firmer texture. 
Spirulina tenutssima is occasionally found rather sparsely mixed in the 
composite mats of green and blue-green alge, seen in some parts of this belt, 
though both these and the Spirulina are more characteristic of higher levels. 
BACILLARIALES, 
Of the diatoms occurring on the beach and Marsh between 1.5 and 6.5 feet 
only two species have been noted as abundant. These are Melosira and 
Pleurosigma. 
Melosira occurs usually sparingly in the mats and felts on pebbles or stalks 
of Spartina between the 4 and 6.5 foot levels. The filaments of Melosira found 
here are of many cells in length and are apparently in living condition. It is 
not certain whether this species propagates freely here or whether the filaments 
found arise from bits broken off from the epiphytic tufts on the Zostera. 
Pleurosigma angulatum is found scattered over the felts of other alge at 
and just above the 6-foot level. 
CHLOROPHYCEZ. 
Of this group at least a dozen species have been found in the mid-littoral 
Marsh, some being quite abundant. (See plate vir.) 
Chetomorpha aerea forma linum, which we have mentioned as present on 
the bottom, is also found frequently mixed in the mats of Rhizoclonium about 
the Spartina stalks from 6 feet downward along the whole shore of the harbor. 
Cladophora (expansa?) occurs abundantly about all four sides of the harbor 
from the 4-foot up to the 6.5 or even %-foot level. In this habitat this species 
