63 



Reeve's 1. novae-hollandiae (1847) must therefore be 

 relegated to the place of a synonym of Sowerby's I. evanida 

 (1840). 



But while the majority of adult specimens conform to 

 Sowerby's description of /. evanida, a few show continuous 

 ribbing throughout the pleural area and right across the dorsal 

 area, a characteristic that heretofore has been considered the 

 most marked distinguishing feature of I. australis, Sby. This 

 ribbed character is shown in one of the Port Arthur shells 

 only 63 mm. long, and one collected by Dr. Torr at Stanley, 

 North Tasmania, only 55 mm. long, while a Port Arthur shell 

 80 mm. long is typical evanida. 



Then, again, while the form in which the pleural area is 

 ribbed right across is rare in South Australia, Dr. Torr has a 

 specimen he informs me that he collected at Tungkallilla, on 

 the South Australian coast open to the ocean, which is 45 mm. 

 long, but is as strongly ribbed as the Port Jackson specimens 

 of /. australis, and from the same locality a shell 33 mm. long 

 showing as extensive ribbing in proportion to* its age and size. 



But none of these strongly-ribbed variants from type from 

 South Australia, Tasmania, and Victoria that I have examined, 

 show the widely -spaced, prominent ribbing in the lateral area 

 that is present in the Port Jackson shells. On the other hand, 

 shells from Bulli, N.S.W., a good way south of Port Jackson, 

 show a departure from the typical shell from that port in that 

 the anterior valve is similar to the form occurring in the 

 southern States, the ribbing being less pronounced and closer 

 together. 



To sum up the evidence before us in regard to specimens 

 from the States of South Australia, Tasmania, and Victoria : — 



(1) No really adult shells are absolutely smooth in the 



pleural area as described by Reeve. 



(2) That the age at which the shells from any of the 



localities referred to begins to show partial ribbing 

 varies very much. 



(3) That living alongside with I. evanida,, Sby., in some 



parts of each of the three States named are shells 

 in which the ribbing is as extensive, and closely 

 approximates to, the Port Jackson, N.S.W., 

 I. australis, Sby. 



(4) That all the adult shells from the southern States show 



a similar sculpture in the lateral area, viz., ribbing 

 nearer together and less pronounced than is the 

 case with the Port Jackson shell, but vary as 

 regards the sculpture of the pleural area, from 

 faint, broken ribbing over a portion of the area, 

 to, in a limited number of cases, the area completely 

 covered with strong ribbing, as in I. australis, Sby. 



