542 Transactions. 
antipodum, which is abundant on mud. Crustacea include the crabs 
Heterograpsus sexdentatus and Heterozius rotundifrons, the shrimp Betaeus 
isopod 
acquimanus, and a fast-running isopod. 
GRASS-WRACK FORMATION. 
Zoslera beds occur on mud-flats between tides in harbours throughout 
New Zealand. Tliey extend from about half-tide mark down to below 
rm of the leaves, evidently an adaptation to tidal currents. According 
to Ostenfeld (Rep. Danish Biol. Sta., vol. 16, p. 6, 1908), the leaves contain 
air, which causes them to stand upright in the water. 
There is an extensive literature on Zostera. Here will be noticed that 
Southern describes an intertidal formation in Clew Bay, west of Ireland, 
and Hedley gives an account of the Zostera association between tides in 
~ 
Sydney Harbour. 
the most part, quite wet while the tide is out. The rhizomes of Zostera 
nana, with their. roots and leaf-bases form, just below the surface, a dense 
mat of vegetation, which effectually holds the mud together. Above the 
surface their short leaves, 10-15 em. long, practically cover the ground, 
which is thus fairly well protected from the scour of the tide. Crabs’ burrows 
the surface, but only small specimens, with shells 23 mm. and under in 
length, were observed. The osterior portion of the valves was eroded 
and stained. Nucula hartvigiana appears to be common, also small examples 
of Tellina liliana. 
Molluscs occurring on the surface of the mud are Cominella adspersa, 
C. maculosa, C. lurida, Cerithidea bicarinata, and small Neothais succincta, all 
with the shells more or less eroded. On the Zostera leaves are herbivorous 
gasteropods— T'urbo smaragdus, Monodonta subrostrata, and Cantharidus 
tenebrosus huttoni. . Certain animals habitually rely on molluscs to carry 
them about. Sea-anemones attach themselves to the shells of Antigona 
stutchburyi. Turbo smaragdus here carries large loads: there may be a 
8 
oidea, or algae containing 
Dardanula olivacea. The Crepidula itself is sometimes covered with 
Vermilia and Spirorbis. Amaurochiton glaucus is found on the larger 
shells of Cominella adspersa and Neothais succincta. 
