BY H. LEIGHTON KESTEVEN. 



5 



As a rule A. Jimhriahim is flatter than A. tentoriforme. 

 Although the above characters are the most constant, none of 

 them can be implicitly relied on. I have seen A. fi^nhriatum with 

 a concave base and A. tentoriforme without any sign of a tooth. 

 There is, however, one recognition mark to which one may pin 

 one's faith — the operculum ; this will be always found reliable. 

 The differences are shown by my figures. 



?(^&^- 



O 



A. fimhriatnm A. tentoriforme. 



Fig. 3.— Adult operculum. Fig. 4. — Adult operculum. 



The radulse are almost exactly alike; if there is any difference it 

 is that the marginals of A. fimhriatum are larger than those of 

 A. tentoriforme. 



It is more than probable that both 

 species occur in South Australia and 

 Tasmania, though only A. fimhriatum 

 has been recorded from there. 2\ 

 georgianus was obtained at King 

 George's Sound, S.W. Australia. 



These two species would seem to 

 present an exception to the rule that 

 distinctive characters are inherited 

 earlier in successive generations, for 

 by that theory A. fimhriatum, should be 



descended from discoidal ancestors, whilst y1. tentoriforme should 

 come from trochiform ancestors, and such a divergence of 

 ancestry would be manifested in anatomical characters. 



A. tentoriforme. 

 Fig. 5. — Dentition. 



Postscript. — Two outline drawings of the apices of Columhella 

 seraiconvexa, Lamarck, and C. australis, Gaskoin (figs. 6-7), shown 



