THE NAUTILUS. 125 



forms of Atopos, Hadra, Chlorilrs and Pupuina ; Atopos prismaticus 

 of Papua claiming affinity with A. australis of Queensland; Hadra 

 broadbenti with H. fraseri ; Chloritis chloritoideK, with C. porteri ; 

 and Papuina naso with P. macgillivrayi. The species actually 

 common to both regions are few ; B. macleayi inhabits both countries, 

 T. annula only finds a place in the Queensland catalogue by court- 

 esy, while Pupa pedicula, S. gracilis, T. ceylonica, T. valida, and 

 Leptopoma vitreum are widespread throughout Polynesia. From 

 these premises it may be deduced that the Queensland mollusk fauna, 

 though isolated sufficiently long to have lost specific identity with 

 that of Papua, has nevertheless been derived from it. 



The shallow sea of Torres Straits now severs this continent from 

 the adjoining island. Were its bed raised but seven fathoms, the 

 two countries would be united, while an elevation of ten fathoms 

 would form a wide bridge between them. When the marine life 

 east and west of Torres Straits is better known, it will be of interest 

 to observe whether the influence of an ancient isthmus is still visible 

 in any divergence between the fauna 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 fresh-water 

 mollusca of tropical S. Australia [North-central 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 mollusk 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 impassible 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. 



