460 MOLLUSCA OF MAST HEAD REEF, QUEENSLAND, I., 



Mast Head Reef, though sharing many features of the Coral' 

 Atolls of the Central Pacific, yet differs widely in detail, a 

 difference due to the circumstances under which each arose. 

 Typically the Central Pacific Reefs spring from abyssal depths, 

 will have, have, or had a deep central lagoon, according to the stage 

 in evolution attained. Their dry land is always greatest on the 

 windward side and is built close to the reef edge. 



Mast Head is a tabular mass set on a platform of about 20 

 fathoms, common to the archipelago. Its lagoon is almost 

 obliterated; the depth at low water being expressed in inches 

 instead of fathoms. The dry land is massed on the leeward side 

 and heaped far within the margin of the reef. 



In the Central Pacific the dry land of the atolls seems to have 

 originated in the spasmodic action of hurricanes, which tear 

 masses off the reef edge and stack them above high-water mark 

 in " hurricane beaches." Between the hurricanes little change 

 occurs. There the tides are small. 



In contrast be it noted that Mast Head lies beyond the 

 hurricane zone, but is subject to the action of enormous tides of 

 a range of 14-15 feet. These tides race over the reef at a rate of 

 two knots an hour, are the chief agents of island building, and 

 operate with more regularity but less violence than the hurri- 

 canes. 



The British marine mollusca were grouped by Forbes and 

 Hanley into those of the Littoral, Laminarian and Coralline 

 zones. An analogous but not parallel division of the Queensland 

 marine mollusca may separate them into inhabitants of the main- 

 land beaches, of the mangrove swamps, and of the coral reefs.* 1 

 The latter fauna, with which we are now concerned, is again 

 segregated into the mollusca of the surf-swept beaches, of the 

 rocky zone from low-water to the mud-line, and of the flat 

 expanse of soft white mortar-like mud beyond the rocks. 



The swift currents that rush through the Capricorn Islands 

 remove the mud. The two upper zones were here only available 



*Tenison Woods, these Proceedings, v. 1881, p. 107. 



