ОтлуЕв.— Marine Littoral Plant and Animal Communities. 497 
the lower limit of terrestrial vegetation, whilst where fresh water flows 
over the rock М. cincta and Notoacmea pileopsis are found by the side 
of fresh-water algae. The varied forms of animals and plants occurring 
between tide-marks may be accounted for by the wide range of conditions 
prevailing there as the result of the regular alternations in the amount of 
water supplied by the semi-diurnal rise and fall of the tide. Within this 
belt are found representatives of all classes of organisms, ranging from 
fishes to unicellular algae, and the one character by virtue of which they 
are classed together is their ability to endure periods of complete sub- 
mergence as well as to tolerate exposure to the air for a portion of each day. 
The richest community in the intertidal belt is that of large brown algae, 
and extends between the limits of low-water, spring and neap tides. This 
is really only the upper margin of an extensive submarine forest, but 
being represented above low-tide mark it will be included in the present 
account. 
In defining ecological provinces in the littoral belt, all authors I have 
consulted, with the exception of Southern and Hedley, divide it into hori- 
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plants and animals present. These depend, in the first place, on the nature 
of the substratum—whether rock on the one hand, or sand and mud on the 
other. Hence Southern (1915, p. 16) speaks of a “ microlithic " and а 
within the wash of storm-waves. These I call the high-tide, mid-tide, 
and low-tide belts, and use the word “littoral” as applying to all that 
portion of the shore between the highest wash and the lowest level of spring 
ides. Iam unable to adopt the terminology of any other author, as none 
to whose work I have had access defines his belts in terms of tide-levels. 
` The belts of Johnston and York (1912, р. 2) appear to correspond most 
nearly with those here defined. 
The upper limit of the mid-tide belt corresponds roughly with the upper 
Above this line many marine animals are never found. onversely, many 
littoral animals keep strictly within tide-marks. The animals confined 
