PRESIDENTIAL ADDRESS. 



57 



mm 



lands is the genus Thais, in which a large and solid shell is 

 fortified by various devices. On the coast of New South 

 Wales this genus is represented by a common species, T. 

 succincta, which is yellow, ovate and about three inches 

 long (fig. 19). In sheltered waters this shell has a com- 

 paratively smooth surface, but on 

 exposed beaches it develops mas- 

 sive rings. So prominent are these 

 that Chemnitz, an eighteenth 

 century conchologist, proposed, on 

 account of the intervening fur- 

 rows, to call it the 'Oartrut ' shell. 

 Both sudden blows and steady 

 pressure can be borne without 

 injury by this belted armour. A 

 related mollusc, Drupa margin- 

 alba, common in the rock pools, 

 attains the same end by girdles of 

 projecting knobs. 



Fig. ]9. Thais succincta, a 

 whelk fortified by armour 

 belts to endure the violence 

 of the surf. 



The ocean reef is as regularly differentiated into horizons, 

 as the estuary. An upper zone between the high water of 

 spring and neap tides is inhabited chiefly by transition 

 amphibious forms such as the periwinkles already mentioned, 

 Tectarius and Melaraphe, also the barnacles Chthamalus 

 and Tetraclita. A median zone between high and low 

 neap tides is characterised by Galeolaria. In a lower zone 

 between low neap and low spring tide Cynthia is a dominant 

 and typical form. 



The intertidal vegetation of the ocean reef has for its 

 most conspicuous form Hormosira banksii, which is rarely 

 absent and which may develop into a dense sward, a pure 

 formation, extending for acres (Plate II, fig. 4). It is now 

 proposed to call such a field an " hormosiretum" and to regard 

 it as comparable in ecological importance to the zosteretum . 



