436 ^Ir. C. Hedley on the Range o/" Placostylus. 



the south, and from Lantliala, Fijis {P. morosus), in the east, 

 to Lord Howe Island {P. bi'varicosus) in the west; and, so 

 far as is yet known, is distributed as follows : — New Cale- 

 donia, 34; Solomons, 16; New Hebrides, 3 ; Fiji, 16; New 

 Zealand, 1 ; Lord Howe, 1. Tiic area of distribution of 

 Placostylus corresponds generally to that great arc of volcanic 

 activity which stretches across the south-west Pacific from 

 the Solomons through the New Hebrides to New Zealand. 

 On either side of this earth-wave extend banks to New Cale- 

 donia, Fiji, and Lord Howe, indented by abyssal gulfs. Tiiis 

 plateau, which for want of a better name I will call the 

 j\1elanesian Plateau, is probably circumscribed by the 

 1300-fathom zone, and probably the various archipelagoes 

 upon it are connected by comparatively shallow banks ; but 

 the fragmentary knowledge we 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 Fijis a sounding of 2G00 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 Placo- 

 stylus. They are large, densely wooded, with a warm, moist, 

 and equable climate. The distance from their western neigh- 

 bours is no greater than from the latter to the 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 mollusk fauna comparable 

 to that of Tahiti, while the mollusk 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 Keef. 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-iatliom level may lie, not 

 between the Great Barrier Keef and New Caledonia, but at 

 the liead of the gulf between the Loyalties and the New 

 Hebrides. 



