THE PIDDOCK. 59 



there is rock, and its dimensions and general appearance vary 

 together with the locaHty. The chalk cliffs, which bound so 

 many miles of our coast, are thickly studded with the burrows 

 of the Piddock, which takes up its residence as high as the mid- 

 water zone of the coast, and in some places is so plentiful, that 

 the hand can scarcely be laid upon the rock without covering 

 one or two of the holes. 



The shell itself is extrem.ely fragile, and of a rather soft tex- 

 ture, and its outer surface is covered with ridges, that sweep in 

 the most graceful curves from the hinge to the edge, and bear 

 some resemblance to the projections upon a file. Yet practical 

 naturalists have proved that, by means of these tiny points and 

 ridges, the Pholas is able to work its way into the rock ; for not 

 only can a similar hole be bored by using the shell as a bradawl 

 is used to pierce wood, but the creature has actually been 

 watched while in the act of insinuating itself into the chalk rock, 

 a feat which was performed by gently turning the shell from 

 right to left, and back again. 



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 mosdy the richest in specimens, but even the hard 

 limestone formations are penetrable by the fragile shell of the 

 Pholas. It has been well remarked, that the size of the Pholas 

 and the 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 comparatively small, and the surfaces are rubbed nearly 

 smooth. The very worst examples, however, are those which 

 are found in gritty rocks, interspersed with pebbles. The shells 

 that have burrowed into such substances are dwarfed, abraded, 

 and often misshapen, and are valueless except to the physio- 

 logist. 



Perhaps the Date Shells are even more powerful as bur- 

 rowers than the molluscs which have just been mentioned One 



