28 C. HEDLEY. 



these gain a footing, but perish when the stream swings 

 off shore. 



Strombus luhuanus, a common and conspicuous shell on 

 coral reefs was once abundant at the Bottle and Glass rocks. 

 Then it disappeared from May 1865 till April 1896 when it 

 again made its appearance. 1 A living specimen of the 

 tropical Bursa mammata, Bolten (= venustula, Reeve) 

 was found alive in the Harbour by T. Rossiter, but in the 

 forty years that have since elapsed, it has not once been 

 seen again. 



After the passage of years changes occur in the distri- 

 bution of species. Thus collectors constantly note that 

 some particular species has become more or less abundant 

 than formerly. At the time of its description (1891) 

 Eulimella monili forme, 2 was quite abundant on Ruppia at 

 the mouth of the Curl Curl Lagoon. Since then I have 

 searched in vain for a single specimen. Local history does 

 not extend back sufficiently far to register many such 

 changes, so the following instances are quoted from abroad. 

 The common periwinkle of the European market, Littorina 

 litorea, was unknown at Woods Hole, Massachusetts, before 

 the year 1876. 3 Migrating from the north this periwinkle 

 took possession of a new field, and so thrived there that in 

 thirty years it actually became more abundant than any 

 other shell on the beach. 



In 1850 Prof. C. B. Adams examined and catalogued the 

 mollusca of the beach at Panama, noting the position, and 

 relative abundance of each species. Sixty years afterwards 

 this area was re-examined by Mr. E. W. Presbrey, 4 who 

 observed that, " Everywhere in the region covered by 



1 Brazier, Proc. Linn. Soc. N.S. Wales, xviii, 1894, p. 432, and Hedley, 

 op. cit., xxi, 1896, p. 88. 



2 Hedley and Musson, Proc. Linn. Soc. N.S, Wales, xvi, 1891, p. 247. 



3 Verrill, Am. Journ. Sci., (3), xx, 1880, p. 251. 



4 Presbrey, The Nautilus, xxvi, 1913, pp. 121 - 125. 



