64 



Conclusion. — On first commencing this inquiry I expected 

 that the conclusions come to by Mr. Bedley in his very suggest- 

 ive paper on "The Effect of the Bassian Isthmus upon the 

 existing Marine Fauna," Proc. Linn. Soe. N.S. Wales, xxviii., 

 1903 (1904), would be amply supported by the distribution of 

 the genus under review. That /. australis, Sby., would prob- 

 ably extend down from New South Wales to the Gippsland 

 coast, and as the western side of the lost isthmus is reached, 

 say the west side of Wilson Promontory, be replaced more or 

 less abruptly by I. evanida, Sby. But in view of the fact that 

 juvenile shells of both are equally smooth and inseparable, and 

 the extreme variability of the southern form, we must conclude 

 that all Australian representatives of this genus have a common 

 ancestry, that the southern shells only represent a race, and do 

 not warrant more than subspecific rank, and may be even 

 relegated, as a result of future investigation, to the subordinate 

 place of a mere variety. I conclude that the strongly-ribbed 

 shells that occur (although but rarely) with the smoother forms 

 in the south, are the progenitors of that section of the species 

 that spread northward ; while the more variable but smoother 

 form spreads westward. The evidence points to the distribution 

 of this genus in Australian waters having taken place since the 

 breaking down of the Bassian Isthmus and the existence of the 

 present straits. We must, therefore, conclude that the 

 known Australian representatives of the genus Ischnoradsia 

 are limited to one species and one subspecies, Ischnoradsia 

 australis, Sowerby (1840), and Ischnoradsia australis evanida, 

 Sowerby (1840), the former inhabiting New South Wales, and 

 the latter the States of Victoria, Tasmania, and South 

 Australia. 



Note by W. L. May, Sandfobd, Tasmania. 



Having been more or less associated with my friend Edwin 

 Ashby in his investigation into Ischnoradsia, I am thoroughly 

 in accord with his treatment in the present communication. 

 From a superficial glance at specimens from the various Aus- 

 tralasian localities, they appear as one species, in shape, size, 

 and colour, and this impression is confirmed by an examination 

 of the girdle, which is practically the same in all. 



To maintain the several separate species, viz., australis, 

 evanida, and novae-hollandiae , we have to rely solely on the 

 absence or presence, in varying degrees, of the longitudinal 

 sculpture ; and as this can be shown to be quite inconstant in 

 the southern and western shells, and as some of these approxi- 

 mate rather nearly to the Port Jackson form, it seems necessary 

 to treat them all as members of one variable species, but for 

 convenience to maintain evanida for the southern and generally 

 smoother form either as a subspecies, which is perhaps 

 preferable, or as a variety. 



