BY C. HEDLEY, 337 



occupying somewhat the position of the Caledonian-Barrier bank. 

 This theory is totally opposed to the distribution of t\\e Placostylus 

 in particular and of the Melanesian mollusc-fauna in general. 

 "Were it true, then Lord Howe, the furthest western outpost of 

 the Melanesian Plateau, would be tenanted by forms bearing some 

 resemblance to Queensland mollusca. 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 east, then Placostylus would have been replaced by Hadra ami 

 Chloritis, while Pupina and Relicina 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 contemporaneous 

 whole, yet into larger fragments, which, temporarily 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 Melanesian 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 distribution 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 portion are usually tree dwellers, 

 the shell of a light sti'ucture and sometimes brilliantly coloured. 

 Between New Zealand and Fiji a line of soundings has been 



