98 
ANIMAL LIFE ON THE 
nor could I detect it in their dead bodies even up to the 
putrid stage. 
The creature's mode of performing its 
ROCK-BORING OPERATION 
was long a disputed point amongst naturalists. Some 
maintained that the animal secreted an acid which had the 
power of dissolving stone, and other substances in which 
the excavations are made ; others maintained that the edges 
of the mantle and foot were the instruments employed, 
which were said to have crystals of flint thickly deposited 
in their substance ; others held that currents of water thrown 
upon the stone by a vibratile cilia with which the creature 
was thought to be provided was the agent ; lastly, it was 
maintained that by the animal rotating on its own axis, the 
rough rasp-like points, produced by the transverse and 
longitudinal lines of the shell acting upon the stone, ground 
the particles into po^ der, which was carried away from the 
interior by injections of water thrown in by the creature 
itself, and this latter theory of the mode of action is generally 
allowed to be the correct one. John Harper, one of the very 
first discoverers of the true mode of action, after describing 
the expansion of the creature's body in placing the valves 
in boring position, says — " The next act, on the part of the 
animal, is to place his foot firmly at the base of the hole." 
Now, while it is only just to say that Harper was a keen 
observer and a true interpreter of the creature's movements, 
I venture to say that in this assertion he has made a mistake. 
Suppose the foot is placed at the base of the hole, the valves 
of necessity would be obliged to scoop round the foot; 
consequently, where the foot rested would be left untouched, 
and a core in the interior would certainly be the consequence. 
Looking at the construction of the tool — for the valves are 
