44 C. HEDLEY. 



In proportion to the shell this animal is capable of enormous 

 extension and quite conceals the shell when outstretched. 

 If hooked out of its burrow the fore foot doubles on the 

 hind as the animal is slowly swallowed by the shell, finally 

 the operculum, closing in on the snail, reveals the dull red 

 patch of callus by which the species is recognised. When 

 extended and in action, a thin spade, like a disk of a 

 planarian, cleaves a way through the sand. This fore foot 

 is then brought up like a mask before the shell, over its 

 edge waver a pair of blind lash-like tentacles. On either 

 side the mantle may be puckered to a pipe, an improvised 

 inhalent or exhalent siphon. From behind, the elastic 

 operculigerous lobe is drawn, like a hood, over the spire of 

 the shell, and then forward till it underlies the mask. In 

 the rear the hind foot spreads a thin disk like that in front 

 for a shell's breadth behind. It seems to prey on the 

 bivalve Spisula parva, small specimens of which I have 

 found in its grasp. Copulation occurs when submerged 

 under the sand. Quantities of this mollusc are consumed 

 by sharks and rays as evinced by the undigested opercula 

 in their stomachs. 2 



The sand in which Polinices burrow is finer and more 

 even than that where Donax live. Both the Polinices 

 and the Area figured in this paper were obtained from the 

 west side of the sand spit of Middle Harbour. I find that 

 a sample from this locality had grains averaging 0*3 mm., 

 with 0*5 mm. for especially large grains, and 0*2 for especi- 

 ally small ones. The Heart-Urchins, Echinocardium and 

 Maretia inhabit a much lower horizon in the sheltered- 

 sand-zone. 



The most important biological feature of the estuary is 

 the tidal forest, which in the latitude of Sydney (33° 50') is 

 restricted to two kinds of mangroves, Avicennia officinalis 



2 Waite, Eec. Aust. Mus., iii, 1899, p. 134. 



