BIVALVES AND CUTTLES. 289 



estuarine waters are available. This is true to a less extent of other 

 sand- or mud -inhabiting forms which are naturally rare off rocky 

 coasts, and abundant on sandy beaches. But as sand and mud are 

 derived from rocks, it will be found that even neighbourhoods which 

 seem to be exclusively rocky exhibit somewhere stretches of sand 

 haunted by the common bivalves. A good example is the long stretch 

 of sand in the vicinity of Woolacombe, on the north coast of Devon, 

 where are to be found quantities of shells apparently absent from the 

 neighbouring rocky beaches. Similarly, the species of Pholas to be 

 found vary with the composition of the rocks, for many species stick 

 to one particular kind of rock. As to further detail, we may notice 

 that Lima Mans can only be expected between tide-marks in the 

 extreme South, and does not occur on the East Coast. On the other 

 hand, the brown Cyprina islandica is a Northern form, diminishing 

 in abundance as one passes southward. As regards the relative 

 abundance of the others, it should be noticed that in the vicinity of 

 fishing- villages the abundance of the shells of a particular species on 

 the beach depends largely on the fact that the species is used as bait. 

 The habits of the fishermen in regard to the bait used differ much at 

 different parts of the coast, so that a great accumulation of shells of 

 Cardium, Afactra, Mya, Cyprina, or Lutraria. at different parts is 

 not in itself a proof of the predominating abundance of the particular 

 mollusc. 



