THE PIDDOCK. 
59 
there is rock, and its dimensions and general appearance vary 
together with the locality. 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 extremely 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 mostly 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 
