BY C. HEDLEY. 877 



Melbourne zoologists ha,ve frequently expressed to me their 

 surprise at the difference between the fauna they find on the 

 shores of Sydney Harbour and that they know at home. 



It has occurred to me that the break in the marine molluscan 

 fauna, which happens, as we know, somewhere between Twofold 

 Bay and Western Port, or, as I suppose, at Wilson's Promontory, 

 is associated with the vanished Bassian Isthmus. 



Granted tw^o propositions, to be considered later, viz., that the 

 Bassian Isthmus existed, and that Tasmania then stretched 

 further to the south; migration of marine forms from east to 

 west, that is to say along isothermal zones, would be interrupted. 

 To regain the accustomed temperature, an individual or species 

 travelling east from the Great Australian Bight would require to 

 double the south cape of Tasmania. At the present time this 

 would mean the enduraiice of a low temperature. But at that 

 time the prolongation of land to the south meant to the wanderer 

 a still low^er temperature. For we may fairly postulate that 

 though the absolute positions of the zones of temperature might 

 have varied in the past, yet the relative proportion of so many 

 degrees of higher latitude to so many degrees of greater cold 

 doubtless remained unchanged. 



The check low temperature opposes to migration has been 

 clearly expressed by Dr. W. H. Dall as follows : — " The tempera- 

 ture limits of many species are more sharply defined on the side 

 of cold than on that of heat. The difference between 45'^ and 40"^ 

 F. may absolutely check the distribution of a species which would 

 find no inconvenience in a rise of temperature from 45'^ to SO'^. 

 It is probable that this is connected with the development of the 

 young rather than the resisting powers of the adult mollusc.""^ 



The union of Tasmania and Australia has been discussed by 

 Mr. A. W. Howitt,t who points out that between Wilson's Pro- 

 montory in Victoria and Cape Portland in Tasmania, by way of 

 Flinders Island and the Kent Group, the greatest depth is 32 



* Dall, Bull. Mus. Comp. Zool. Harv. Coll. xii., p. 180. 

 t Howitt, Rep. Austr. Ass. Adv. Sci. vii., 1898, pp. 723-758. 

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