74 C. HEDLEY. 



uniform purple, but in shelter it becomes thinner, larger, 

 smoother, feebly denticulate and coloured with radial stripes 

 of purple-black and buff. Since it extends north to Japan, 

 but fails to reach Tasmania, it is evidently of tropical origin. 



In the surf it is scattered and single, but in the estuary it 

 packs together in a continuous reef. (Plate IV, fig. 8). 

 Saville Kent has illustrated the far greater masses, four or 

 five feet thick, built by this species in Moreton Bay and 

 Port Curtis. x He draws attention to the fact that the zone 

 of most luxuriant development coincides with half tide 

 mark. Probably it spawns most freely in the saltest water, 

 but grows quicker and larger in water of less salinity. 

 Since the fresh water, that reduces the salinity, carries 

 with it abundance of food material from the land, it may 

 be thus not preferable in itself, but endured for the sake of 

 the accompanying advantages. 



Those oysters that lie on the mud, are apt to suffer from 

 the invasion of a small worm, Leucodore ciliatus. To pro- 

 tect itself from this commensal, the mollusc excludes the 

 worm by a partition wall, a process which, if repeated, 

 brings exhaustion and destruction. 2 



In the vicinity of Sydney the oyster zone is followed on 

 the lower side by a dense growth of mussels, Brachyodontes 

 hirsutus, matted together in a felt of epidermis and byssus. 

 Indeed on the estuarine rocks, Ostrea and Brachyodontes 

 hold positions corresponding to that of Galeolaria and 

 Cynthia on the ocean beach. 



Mussel beds never form so important a feature in our 

 beach scenery as they sometimes do abroad, as for instance 

 on the coast of Normandy. 3 But on the Tasmanian beaches 



1 Kent, Great Barrier Eeef, 1893, p. 254, pis. 39, 40; Naturalist in 

 Australia, 1897, p. 249, pi. 42. 



2 Whitelegge, Rec. Austr. Mus., i, 1890, p. 41. 



3 Guerin, Bull. Mus. Oceanograph, Monaco 67, 1906, p. 14, pi. 1. 



