SHORES OF THE CLYDE AND FIRTH. 
99 
simply the creature's boring tool — as already mentioned, 
and as shown in illustration (p. 96), the lower portion will 
be seen to be somewhat triangular in shape. 
Well, placing the outer edges of this portion in line, we 
have a replica of the counter-sinking drill, the exact work for 
which it is designed. This simple fact in itself is sufficient 
to set the enquiring mind into thinking and observation ; 
and it is very soon discovered that, in order to give the drill 
its proper position to work in, instead of the foot being placed 
upon the base of the hole it is placed upon the side, and then 
the lateral movement required is more easily accomplished. 
From an inspection of the holes in the stone, it will be 
observed that the top or entrance is not exactly spherical. 
Why this should be found from a rotating movement seems 
at first somewhat inexplicable, but as has been already 
observed, the top portion of both valves are oval-shaped, 
and these influencing the shape of the syphon-tube, particu- 
larly near the base, which, when thrown out from time to 
time, shapes the stone accordingly. The interior again is 
sometimes marked with lines and little ridges, but this is 
due to the fact of the creature's valves having considerably 
grown during an inactive period; and when the new portion 
of this chamber is completed, the top of the enlarged valves 
striking against the base of the former chamber, when the 
syphon is thrown out for feeding purposes, leave the lines 
and irregularities observable within. In their boring progress, 
should the angle at which they began lead them into the 
chamber of another, as is often the case, no stoppage is 
made. When the growth of the shell demands more room, 
the boring is renewed, even though it should be through 
the body of a neighbour. How the creature gets quit of 
the particles rasped from the sides and bottom of its 
chamber is a subject on which naturalists widely differ. 
