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The Museum Gazette 



mussel of our rivers and canals (Dreissensia polymorpha) should 

 be compared with the common edible marine mussel. Their 

 affinities will be at once apparent to the most casual observer. 

 In brackish water at the mouths of tidal rivers intermediate 

 forms may be observed. They are quite distinct from the true 

 marine molluscs in having lungs instead of gills. 



The variations in the shape of shells are directly related to 

 the life-habits of their occupants. The Limpets frequent 

 rocks exposed to the full onslaught of the waves, they have 

 almost flat shells which are entirely open below. The large 

 foot of the animal acts like a sucker, it is very strong and 

 possessed of great adhesive power. 



; The common dog-whelk (Purpura lapillus) has a compara- 

 tively long spire and small mouth, characteristics which are 

 especially well marked when occurring in sheltered waters, such 

 as the Solent. On the other hand, the shells of this species 

 from rocks, at Land's End, the Yorkshire coast and other 

 places fully exposed to the wash of the waves, have a short 

 spire and larger mouth ; in other words they approximate, 

 though slightly, to the limpet type of shell. 



This species also illustrates the remarkable variation in 

 colour often observable in shells of one species. The visitor 

 to Newquay will observe that there the shells of the dog- 

 whelk are strongly banded. These banded forms are con- 

 sidered by some writers to show a tendency to protective 

 coloration, as they frequently occur on veined rocks. 



The Rev. A. H. Cooke remarks 1 that " Protective color- 

 ation is not uncommon among the Mollusca. Littorina 

 pbtusata is habitually found, on our own coasts, on Fusus 

 vesiculosus, the air-bladders of which it closely resembles in 

 colour and shape. Helcion pellucidum, the common British 

 " blue limpet," lives, when young, almost exclusively on the 

 iridescent leaves of the great Laminarise, with the hues of 

 which its own conspicuous blue lines harmonise exactly. In 



1 "Cambridge Natural History." Molluscs and Brachiopods, p. 69. 



