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PACIFIC SCIENCE, Vol. IV, October, 1950 
Harbour is absent, though still growing at 
similar levels in the Narrow Neck locality. 
The mat formed by this species serves as a 
trap for sand and silt, which act as a moisture 
reservoir during periods of exposure by the 
tide. 
b. Volsella Consociation (Fig. 8) 
Volsella neozelanicus may be profoundly 
dominant below the level usually colonized 
by Gelidium. This suggests that the upper 
limit of Volsella is raised through association 
with the red alga, the thallus of which acts 
like a sponge in conserving moisture. 
The barnacles Chamaesipho columna and 
Elminius modestus and the encrusting Ralfsia 
verrucosa are subordinate members of this 
community. 
3. Chamaesipho — Elminius — Saxostrea 
Association (Fig. 11) 
Vertical range: 10.2-3.6 feet. Between 
M.H.W.M. and M.L.W.N. 
Exposure: 92-21.3 per cent. 
Chamaesipho columna (d) 
Saxostrea glomerata (d) 
Scytothamnus australis (Id) 
Enteromorpha procera f. minuta (a) 
Splachnidium rugosum (f) (summer) 
Pylaiella novae-zelandiae (f) 
Caulacanthus spinellus (o) 
Gelidium caulacantheum (o) 
Bangia vermicularis (o) (winter) 
Elminius modestus (d) 
Elminius plicatus ( Id ) 
Ralfsia verrucosa ( Id ) 
Volsella neozelanicus (a) 
Sypharochiton pellis-serpentis (f) 
Lepsiella scobina (f) 
Cellana radians (f) 
Cellana ornata (o) 
Urospora penicillaeformis (1) 
( autumn ) 
This association, which is included by Oli- 
ver (1923) in his "Shelled Animals Forma- 
tion,” has the widest range of distribution of 
any inter-tidal community in the region and 
is subject to the greatest variety of changes 
in environmental conditions. 
a. Chamaesipho Consociation 
Chamaesipho columna, the smallest of the 
common inter-tidal barnacles, is usually to 
be found forming a greyish- white, horizontal 
belt about the mark of M.H.W.N. At this 
tide level the community is a closed one in 
places of optimum development, but towards 
its upper limit (M.H.W.M.) the individual 
barnacles are more scattered, and species from 
higher associations, e.g., Volsella neozelanicus, 
may become locally dominant. 
b. Elminius — Scytothamnus Fasciation 
Elminius plicatus, the largest of the inter- 
tidal barnacles in this locality, is locally dom- 
inant just above M.S.L., where it may form 
a closed community, usually not exceeding 
1 foot in vertical extent. Chamaesipho co- 
lumna and sometimes Caulacanthus spinellus 
are common epiphytes on E. plicatus. The 
dark brown fronds of Scytothamnus australis 
are the prevailing feature of portions of the 
community, co-dominating with either E. 
plicatus or E. modestus, the latter mainly at 
lower levels just above the coralline turf. 
During periods of emergence the exposed 
parts of the thalli become dry and parched, 
while portions lying directly upon the bar- 
nacles are still quite damp. 
c. Elminius modestus Consociation 
This community may be present at any 
level between M.S.L. and M.L.W.N. Perhaps 
the most striking ecological attribute of E. 
modestus is its ability to exist in muddy water. 
At Narrow Neck it flourishes on the gently 
sloping dip face of the main reef above Coral- 
lina, where at times there is a layer of fine 
silt and mud Vz inch thick. E. modestus is 
equally at home on flat, vertical, or sloping 
faces, whether shaded or exposed to sun, and 
on both upper and under sides of boulders. 
Once established, it can withstand relatively 
strong tidal currents and wave action. It is 
prevented from colonizing all the available 
rock surface within its vertical range by com- 
