Inter-tidal Zonation — CARNAHAN 
43 
merged. This was done for a tidal period of 
360 hours (spring to spring) (Fig. 7A). A 
better average is obtained over a lunar month, 
or better, a lunar year (Fig. 7B). 
The probable significance of relative sub- 
PERCENTAGE EXPOSURE — — —• 
O 10 20 30 40 50 60 70 80 90 100 
A 
PERCENTAGE EXPOSURE 
Fig. 7. Percentage emergence ("percentage ex- 
posure”) for each foot between E.L.W.S.T. and 
E.H.W.S.T. A, for one tidal period (360 hours), Jan., 
1947; B, for one lunar year (1945) (A. E. Beveridge). 
Rate of increase in percentage emergence ("percentage 
exposure”) also shown in each case. 
mergence and emergence lies in the rate of 
increase of percentage emergence with height 
on the littoral (Fig. 7). This rate of increase 
rises to two peaks, occurring between 3.0 
feet and 4.0 feet and between 9.0 feet and 
10.0 feet. These sharp changes in the rate of 
increase might be the basis of thresholds in 
the vicinity of M.L.W.N.T. (3.31 feet) and of 
M.H.W.N.T. (9.22 feet). 
Calculations were made (from the Nautical 
Almanac predictions) of the periods of con- 
tinuous emergence undergone by each level 
above E.(L.)H.W.N.T. in 1947, and of the 
periods of continuous submergence under- 
gone by each level below E.(H.)L.W.N.T. 
The maximum period appears to be im- 
portant in each case. In the case of emergence, 
it is 1 day at 8.5 feet (through missing a single 
tide). It then rises fairly steadily to 9 days at 
10.0 feet. There is a fairly sharp rise to 23 
days at 10.5 feet and to 43 days at 11.0 feet. 
A great increase in the maximum period of 
emergence then occurs, the time being 132 
days at 11.5 feet. Above M.H.W.S.T., a con- 
dition of permanent emergence is swiftly 
attained. Fairly similar behaviour is associated 
with the periods of continuous submergence. 
Here a fairly steady rise to 25 days at 1.5 feet 
is followed by a sharp rise to 131 days at 1.0 
foot. The 0.5 foot level is permanently sub- 
merged. 
The important point would appear to be 
the meaning given to the mean limits of 
spring tides (defined under "'Terminology” 
as the limits of the littoral) by the great rises 
in the maximum periods of continuous emer- 
gence and submergence at M.H.W.S.T. and 
M.L.W.S.T., respectively. Here again the 
levels appear to be thresholds. 
The relatively large increase in maximum 
continuous emergence between 10.0 feet and 
11.0 feet may control the upper limits of 
Chamaesipho columna (10.3 feet) and Elminius 
modestus (10.5 feet). 
Behaviour of Hormosira banksii. In each 
of the above cases, it would seem that 
