BY C. HEDLEY. 695 



isthmus is still visible in any divergence between the faunas 

 inhabiting the two areas. 



Further to the westward, the coasts of Australia and New 

 Guinea again converge, being separated by an arm of the Arafura 

 Sea, which gradually shoals from a central depth of 40 fathoms, 

 and stretches for about 150 miles between Cape Wessel in the 

 northern territory and Cape Valsche on the opposite shore of 

 Dutch New Guinea. 



In the Transactions of the Royal Society of S. Australia, Vol. 

 v., pp. 47-56, Professor Tate enumerates the land and freshwater 

 mollusca of tropical S. Australia ; it is remarkable that whereas 

 a third of the landshells of Papua and a sixth of the landshells 

 of Queensland are operculate, his census includes no operculate 

 landshells whatever. Thus at the remote date when the ancestors 

 of the present Queensland mollusc fauna migrated from New 

 Guinea across the ancient isthmus that I suppose to have bridged 

 Torres Straits, the Arafura Sea appears to have still presented an 

 impenetrable barrier between the two countries. The former 

 elevation of land in this region, if uniform from east to west, 

 may therefore be calculated at more than seven and less than 

 forty fathoms. 



EXPLANATION OF PLATES. 



Plate xxxviii. 



Fig. 1. — Jaw of N. htmsteini. Magnified. 



Fig. 2. —Jaw of G. louisiadensis. Magnified. 



Fig. 3. — Two rows of seven teeth from the centre, and of the eighteenth 



to the twenty-second from the margin, of the radula of N. 



divisa, var. inclinata. Much magnified. 



Fig. 4. — Two rows of thirteen teeth from the centre, and of the twenty- 

 fifth to the thirtieth from the margin, of the radula of M. 

 sappho. Much magnified. 



Fig. 5. — Jaw of G. trdbriandensis. Magnified. 



