47 



The Land and Fresh^water Molluscs of 

 Tropical South Australia. 



By Professor Ealph Tate, Assoc. Lin. Soc, F.G-.S., &c. 



[Kead July 5, 1882.] 



The land snails of Tropical South Australia are confined to 

 the basin of the northern rivers — to that well-marked natural 

 region which extends from the seaboard to the escarpment of 

 the "Desert Sandstone" plateau, inasmuch as no species has 

 up to the present time been recorded from the extensive tract 

 of country lying north of the MacDonnell Eanges, on the 

 verge of the tropics, to within about one hundred miles of the 

 coast of Arnheim Land. 



That the Desert Sandstone, presumably of Miocene age, 

 extended to the seaboard will be readily conceded by those 

 who have studied the physiographic features of the northern 

 part of Arnheim Land ; and therefore, the region of the 

 northern rivers, occupied by metamorphic rocks, is as a land- 

 surface of recent date. The area of the " basin of the northern 

 rivers" is rapidly enlarging by the removal of the "desert 

 sandstone," by issue of water at its junction with the schistose 

 rocks, thus originating the numerous affluents of the several 

 large rivers of this well-watered portion of Xorth Australia. 



The very large number of immigrant plant species and genera 

 in this region points, likewise, to a more modern origin of its 

 flora as compared with that of the plateau of the Desert Sand- 

 stone. 



AYe need not, then, wonder at the paucity of its land snails, 

 whose means of dispersal are so limited, or even at the absence 

 of those genera so characteristic of Tropical Polynesia and 

 JST.E. Australia. The climatic phenomena are, moreover, un- 

 favourable to the establishment of species requiring shade and 

 humidity ; and the absence of land snails, over the area of the 

 " desert sandstone " and the country to the south with which it is 

 physically and geographically connected, must be attributed to 

 aridity of soil and want of shelter in the form of trees or rocks. 

 Nevertheless, it is not improbable that some species will be 

 discovered in those insular-like masses of old rocks constituting 

 the Ashburton, Forster's, and other Eanges. 



