WAYE-CUEREKTS ON SHALLOW-SEA. FAUNAS. 273 



suppose that the varieties best suited to the bottom and locality 

 will survive. The same may be said in the case of the fry being 

 spread over an area affected by wave- and tidal currents com- 

 bined. In this way different varieties will be localized in different 

 places. 



As an example of the difference in relative compression of dif- 

 ferent specimens of G. echinatum, I may instance two individuals 

 from the English Channel and one from Torbay. The thickness 

 in each case was 1"75 inch, and the leugth of the Channel speci- 

 mens 2"175 and 2'4 inches respectively, and that o£ the Torbay 

 specimen 2'1 inches. These examples were selected (out of a 

 small number of specimens) owing to their uniform thickness 

 making them convenient for comparison. There is no reason to 

 suppose they are extreme cases of variation. The difference in 

 penetrative power in these specimens due to difference in form 

 would be considerable. 



The genus Carclium may again serve to illustrate the iuflaence 

 of wave-currents on the local extinction of species. 



In Torbay the most abundant cockles are G. aculeatum and 

 G. tuberculatum, the former having very long spines, the latter 

 having them quite rudimentary. These two species do not appa-^ 

 rently thrive together (when their shells are perfect) iu the same 

 deposit in Torbay, though their habitats are not far apart, the one 

 being soft and muddy sand, the other pure and firm sand. There 

 can be little doubt that a radical change in the character of the 

 two deposits in which these species respectively live would 

 cause their local extinction, by making it impossible for them to 

 offer a successful resistance to their enemies, the wave-currents. 

 "Whilst the two species referred to are locally abundant in Torbay, 

 Gardhcm ecMnatwn, the species most abundant, as a rule, qn the 

 British coasts, and whose spines are of an interrnediate length, 

 is much less common ; for though often abundant in the earlier 

 stages of growth, even undersized specimens are rare compared 

 with G. aculeatum and G, tuierculatum. I^ either the verv soft 

 nor the very hard sand seems to suit it. 



In past ages, as evidenced by the raised beach on the islet 

 known as the Thatcher, the dominant cockles of the locality were 

 neither G. aculeatum nor G. tuberculatum, but G. ecMnatum and 

 G. edule. There is plenty of geological evidence that in those 

 days the sandstone cliffs were far more extensive than now, and 

 the supply of sand, in consequence, more abundant; and we 



