BY PROFESSOR STEPHENS. 553 



Another age of emergence and amelioration produces the 

 Middle Coal Measures, followed in its turn by the severe interval 

 of the Barren shales. 



These oscillations, however interesting as indications of the 

 regularity in these southern regions, as also of the alternations of 

 climate which are so remarkably illustrated in the history of the 

 Glacial periods of the north, are of no importance to us at this 

 moment. But the next emergence corresponding with the Upper 

 Coal Measures appears to deserve more particular attention. Not 

 only have all the climatic conditions been altered by the reclosing 

 of the cold water channels, but some kind of communication vfith 

 the northern continent has been approximately completed. For, 

 in the rivers of this period, flowing through lands covered with 

 the very same vegetation, and in all other respects apparently 

 just the same as the rivers of the preceding coal-forming epochs, 

 there suddenly appears a quite new arrival from the rivers of the 

 north. For Urosthenes is a Ganoid fish of the Palaeoniscus family, 

 belonging to a genus well-known in the northern Carboniferous, 

 and makes the first appearance of a vertebrate in the Australian 

 freshwaters. 



The Ganoids are essentially freshwater fishes, and though they 

 are tolerant of the brackish water of estuaries, and can doubtless 

 make short voyages by sea from one river mouth to another, yet 

 they are incapable of traversing any considerable tract of salt 

 water, as is indeed shown by the geographical distribution of the 

 surviving members of the order. It is a fair conclusion, therefore, 

 that some means of communication had been at last opened 

 between Australia and Asia. There had been, so far as can be 

 seen, no passage of any organic form from the one land to the 

 other since the period of the Lepidodendron flora, which must 

 have originated in either the Northern or Southern Hemisphere, 

 and whose existence in both requires the hypothesis of a line of 

 communication on the Australian as well as on the American side. 



It would be audacious to argue that the existence of such a 

 bridge (or stepping-stone) between Australia and Asia indicates a 

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