THE PIDDOCK. 



119 



The Pholas burrows to a considerable depth, and if a piece of 

 the rock be detached and broken to pieces by the hammer, it will 

 be seen to be completely riddled with the perforations. Chalk- 

 rock is mostly the richest in specimens, but even the hard lime- 

 stone formations are penetrable by the fragile shell of the Pholas. 

 It has been well remarked that the size of the Pholas and the 



Pholas in Wood. 

 Lithodomus. 



Razor .shell. 



Pholas in Rock. 



sharpness of its markings vary in inverse ratio to the hardness 

 of the rock in which it burrows. From the softest sea-beds are 

 taken the largest and most perfect shells, while those specimens 

 which are obtained from the hard limestone rocks are compara- 

 tively small, and the surfaces are rubbed nearly smooth. The 

 very worst examples, however, are those which are found in grit- 

 ty rocks, interspersed with pebbles. The shells that have bur- 



