Mr. C. Hedley on the Range q/Placostylus. 437 



Wallace, in his 'Island Life,' advances the theory* that 

 Australia and New Zealand were formerly connected by a 

 bridge of dry land occupying somewhat the position of the 

 Caledonian-Barrier bank. Tliis theory is totally opposed to 

 the distribution of the Placostylus in particular and of the 

 Melanesian moUusk fauna in general. Were it true, then 

 Lord Ilowe, the furthest western outpost of the Melanesian 

 Plateau, would be tenanted by forms bearing some resem- 

 blance to Queensland MoUusca. Had the stream of life 

 reached Lord Howe from the north-west instead of from the 

 north-east, then Placostylus would have been replaced by 

 Hudra and Chloritis, while Papina and Helicina would have 

 been substituted for Realia and Omphalotropis. 



The various islands inhabited by Placostylus would seem 

 to have been joined, if not into one continuous and contem- 

 poraneous whole, yet into larger fragments, which, tempo- 

 rarily united, allowed the passage of snails from one tract to 

 another. Should it be proved that the islands occupied by 

 Placostylus are now sundered by deeper channels than that 

 between Australia and the ^lelanesian Plateau, even that 

 would not defeat the argument of their former union and of 

 their eternal separation from Australia. Not the depth but 

 the permanence of the ocean is the real limit to the distribu- 

 tion of forms of life. The geology of the Solomon-New- 

 Zealand arc, imperfectly as it is yet read, shows a most 

 tempestuous record of lands now sunk in the stillest ocean- 

 deeps and anon flung into lofty mountain ranges. The 

 history of the north-east Australian coast exhibits no such 

 vicissitudes, but it appears to have retained its present outline 

 for long ages past. The channel joining the abysses of the 

 Coral and of the Tasman Seas would therefore be more 

 permanent than channels, possibly deeper, intersecting the 

 Melanesian Plateau. 



The genus Placostylus divides itself naturally into halves. 

 The southern portion are inhabitants of New Caledonia, Lord 

 Howe, and New Zealand. Almost all are heavy massive 

 shells, dark in colour, confined to the ground by the mere 

 weight of the shell, and singularly unfitted to cross distant 

 seas by any means that I can imagine. The northern por- 

 tion are usually tree-dwellers, the shell of a light structure 

 and sometimes brilliantly coloured. Between New Zealand 



• " Confining ourselves strictly to the direct relations between the 



plants of New "Zealand and of .\ustralia I think I may claim to 



have shown that the union between the two countries in the latter part 

 of the Secondary epoch .... does sufficiently account for all the main 

 features of the New Zealand flora " (2nd ed. p. 506). 



