336 THE RANGE OF PLACOSTYLUS, 



yet possess of the contour of the floor of the South Pacific does not 

 enable us to trace its margin. 



Eastwards of Fiji, the molluscan fauna indicates the abrupt 

 termination of the Melanesian Plateau. Between the Samoas and 

 Pijis a sounding of 2600 fathoms has been obtained. Significant 

 of this is the absence of Placostylus from Savaii, Upolu or Tutuila. 

 The Samoan Islands appear as well fitted as the Fijian to nourish 

 an extensive series of Placostylus. They are large, densely wooded, 

 with a warm, moist and equable climate. The distance from 

 their western neighbours is no greater than from the latter to tlie 

 groups to the westward, and not to be compared to the spaces 

 between New Caledonia and Lord Howe or New Zealand, which 

 have proved no obstacle to the spread of the genus. Yet the 

 Samoas possess a distinctly oceanic mollusc fauna comparable to 

 that of Tahiti, while the mollusc fauna of the Fijis is as distinctly 

 continental. 



On the westward we learn from the " Challenger " soundings 

 that about the 20th parallel a bank of a maximum depth of 

 1300 fathoms connects the Melanesian Plateau with the Great 

 Barrier Reef. This bank was not actually plumbed, but its 

 existence is inferred from the fact that soundings in the Coral Sea 

 diminished in temperature down to 1300 fathoms, and below that 

 level to 2450 fathoms the thermometer readings were stationary. 

 The inrush of cold water from the Antarctic abyss is therefore 

 stopped by banks, whose lowest depth is 1300 fathoms, hemming in 

 the abyss of the Coral Sea. But the canal whose floor is the 1300 

 fathom level may lie, not between the Great Barrier Reef and 

 New Caledonia, but at the head of the gulf between the Loyalties 

 and the New Hebrides. 



Wallace,in his "Island Life," advances the theory '^•' that Australia 

 and New Zealand were formerly connected by a bridge of dry land 



*" Confining ourselves strictly to the direct relations between the plants 

 of New Zealand and of Australia, .... I think I may claim to have 

 shown that the union between the two countries in the latter {jart of the 

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

 features of the New Zealand Flora" ; ^nd Ed. p. 506. 



