136 BURROWING SHELLS. 



at a height of nearly two hundred feet above the sea, a 

 situation which the tide never washes over, but where 

 the giant waves of the Atlantic throw up pretty con- 

 stantly a feathery spray. This spray collects in pools 

 on the summit of the crag, where it is largely diluted 

 with rain-water; and here, in this unpromising locality, 

 multitudes of Littorina rudis have taken up their abode. 

 The specimens are quite as large as the usual state of 

 the species, but the substance of the shell is nearly as 

 thin as that of a Limneus, especially about the aperture, 

 and the grooves between the spires are much deeper 

 than usual. Still, though changed, the species is easily 

 recognized; nor is there the slightest disposition to 

 pass into L. petrcea. 



I have already, in the chapter on Sands, spoken of 

 some of the general habits and structure of the bivalve 

 Mollusca, the great majority of which live in sandy or 

 muddy places. Some, however, like the Edomites, take 

 up their abode in the rock, and hollow out for them- 

 selves dwellings in it. Such is the Pholas, of which 

 we have several British species, which are often found 

 imbedded in limestone or sandstone rock, though oc- 

 casionally they con- 

 tent themselves with 

 houses of clay. How 

 so frail a shell as Pho- 

 las Candida, which is 

 PHOLAS CANDIDA. as thin as paper, and 



as brittle as glass, is 



able to work its way through hard stone, has long been 

 a puzzle to Naturalists ; some of whom assert that it 



