BY FRITZ NOETLING, M.A., Ph.D., ETC. 253 



R. M. Johnston has kindly supplied the following infor- 

 mation on this point: — 



Four species of fish are identical, viz.: — 



1. Gadopsis marmoratus (the well-known black- 



fish). 



2. Galaxeus attenuatus (the jollytail). 



3. Prototroches marena (the fresh water herring). 



4. Retropora richardsoni (the fresh water whitebait). 



Of molluscs there is 

 Unio mortonicus. 



All these species originally occur only in the northern, 

 but not in the southern rivers of Tasmania (i). The 

 faunistic difference between the northern and southern 

 rivers could not be sharper marked than by the distribu- 

 tion of the genus Unio. 



The hypothesis here promulgated affords the easiest 

 solution of the problem. If we assume that a large 

 stream — the combined Mitchell and Snowy rivers — was 

 running across the peninsula, successively taking up in 

 its course the northern rivers of Tasmania, the road for 

 a migration of the Victorian fauna into the northern 

 rivers is open. Naturally the more mobile fishes found 

 their way in larger numbers to Tasmania than the more 

 slowly moving mollusca. 



It will further be seen that on account of this fresh 

 water fauna no saline lake, such as would result to-day 

 if the sea level were to recede to the 45-fathom level, 

 would have formerly existed. All lakes without discharge 

 are saline, and the Bassian Lake would make no excep- 

 tion to this rule if it came into existence. Though there- 

 fore the Victorian rivers would discharge their water 

 into this lake, its saline nature would certainly prevent 

 the migration of the fresh water fauna from Victoria to 

 Tasmania. It is, therefore, pretty certain that the de- 

 pression in the centre of Bass Strait which would appear 

 as a lake, if the sea receded to the 45-fathom level, was 

 formed after the migration of the Victorian fauna into 



(i) Gadopsis marmoratus has now been imported into several 

 southern rivers. 



