LITHOPHILOUS BENTHOS 31 
of these were detached, only a few dozens of them being attached to mussels. 
The only source that could at this time be discovered for such numbers of the 
plants of Porphyra was the dense colony of them in the Inlet. With so many 
detached plants about the harbor it is interesting to note that they do not settle 
on the bottom to cover large areas, as Ulva does. This is evidently because the 
Porphyra floats and is therefore cast up on the beach instead of settling. 
Porphyra is noticeably free from epiphytes, probably because of its lubricous 
surface. 
In the summer Porphyra is relatively rare in the Inner Harbor, though it is 
still abundant at various points in the Outer Harbor. The plants found in our 
area 1n summer are chiefly on the wharves, between 2 and 4 feet, and their 
distribution at these levels will be noted in discussing the rockweed association. 
The factors determining the distribution of Porphyra are not very clearly 
indicated by its occurrence in the area under observation. The limits noted in 
April and July, 2. e., —1 to +4 feet, are very nearly the limits of distribution of 
the plants found on the open shores of Long Island Sound. From the observa- 
tions here made it seems evident that cool water, stirred by waves or tidal 
currents, furnish the conditions favoring the growth of Porphyra. The upper 
limit seems to be determined by the time of exposure to the air, and the lower 
limit probably by the lack of light due to the turbidity of the water. It seems 
hardly probable that a delicate alga of a single layer of cells can be confined to 
levels above low tide, because of the need of exposure for aeration. 
Hildenbrandwa prototypus, an incrusting species, is more widely spread in 
summer than any other red alga of the Inner Harbor. It occurs on pebbles and 
stones at all levels from mean low water up to 6 or 6.5 feet and wherever there 
is a proper substratum. It grows both in pure salt water and in places where 
the plants may be overflowed for several hours at each tide by fresh water. For 
example, it is found abundantly on pebbles along the channel to the Outer 
Harbor from mean low water up to 1.5 or 2 feet, and on the shoals beside the 
Creek, at 470 east and 625 east at 1 to 2 feet, and, finally, it occurs within the 
present belt on stones of the wharf of the Research Laboratory, and of the 
wharves east of the Inlet at 2,200 north to 2,600 north. The distribution of 
this alga at higher levels and the factors determining its upper and lower limits 
will be discussed in describing the next higher belt of vegetation. 
The remaining ten species of Rhodophycee found on the bottom of the harbor 
are usually represented by few dozens or scores of individuals each. In fact, 
one or more of the species may be entirely wanting in some summers. Some 
of these ten species may develop in situ, on stakes or buoys, or on stones of the 
bottom. Others are rarely found fixed to a stable substratum. More often they 
are found drifting about over the bottom, being either entirely free or dragging 
about with them small pebbles or shells which hold the young plants in place, 
but which are too small to anchor securely the now full-grown plants. The ten 
species to which we have referred are: Agardhiella tenera, Callithammion 
roseum, Ceramium rubrum, Chondria tenuissima, Dasya elegans, Delesseria 
leprieurtt, Gracilaria multipartita, Grinnellia americana, Lomentaria uncinata, 
and Polysiphonia variegata. Of these alge Callithammion and Lomentaria have 
each been found during one season only, when a considerable number of plants 
of each were established in the Inlet, near 2,300 north by 1,175 east, at about 
mean low water. 
