512 Transactions 
Mud. 
Animals.—Some of the adaptations of pelecypods to a muddy substratum 
form an interesting contrast to those of species living in sand. Again 
refer to Hedley’s interesting paper (1915, p. 50), where a ventricose rough 
shell, a small foot, and no siphons are given as characteristics of the 
mud-dwelling Arca trapezia of Sydney Harbour. An exactly parallel case 
is afforded by Antigona stutchburyi of New Zealand muddy estuaries, The 
shell is swollen and but moderately thick, and roughened with concentric 
and radiating ridges; the animal is furnished with a small foot and short 
siphons. But it must be pointed out that these characters are not universal 
in mud-dwelling pelecypods, for Amphidesma australis associated with 
Antigona has a compressed, smooth, solid shell. 
A type of gasteropod shell characteristic of intertidal mud-flats is the 
turreted form. A heavy shell of this shape can be dragged over mud or 
sand, but would be very inconvenient on a rough rocky surface. Cerithidea 
bicarinata is an example. Better instances of this relation could be quoted 
from outside New Zealand—for example, the heavy Pyrazus on mud in — 
Australia, and the more massive Terebra on sandy bottoms in the tropics. 
Plants.—Zostera nana and 2. tasmanica, covering extensive areas on 
sheltered mud-flats between tides, are furnished with rhizomes which spread 
MovEMENT or WATER. 
. Perhaps the most potent of factors in the littoral belt with which the 
inhabitants have to contend is the movement of water. Tidal currents are 
their maximum strength during storms. The power of the surf may. 
gauged by the damage done by heavy seas to breakwaters and other harbour 
inhabiting the intertidal ‘belt appear to be adaptations caused by the motion 
of water. This is shown by their shape, strength, pliancy, and by their 
and plants in the littoral belt in New Zealand will here be described. - m 
(1.) Tent-like Form.—'To offer least resistance to the surf the animal _ 
must be attached to the rock by a broad base, have sloping sides and а i 
fairly low apex. Hence results the tent-like form so prevalent on rocky 26 
coasts, and independently acquired by members of several classes of @ WERTE 
в.) Gasteropoda, —A conical shell with a more or less oval bes. due 
characterizes the members of the Patellidae, Acmeadae, Siphonariidae, аре — 
the genera Tugalia and Gadinia. There is an apical foramen in {зв 
and Puncturella, and an anterior marginal slit in Emarginula. : 
(6.) Amphineura.—In proportion to basal area members o 
offer less resistance to the surf than do the tent-like gasteropods. 
