40 THE SHELL-FISH OF THE COAST. 
against the rock in concentric lines. However this 
may be, it is certain that the hole is very evenly 
cut, and that it is increased in size in conformity 
with the development within the rock of the animal 
itself. 
A rather remarkable form of borer, which was 
first detected within the shells of the oyster at 
~ Tottenville, Staten Island, is the Diplothyra Smithii, 
avery much smaller shell than either of the pholads, 
and perhaps more nearly recalling the true ship- 
worms. The latter (Pl. 2, Figs. 4, 12, 13), by rea- 
son of their depredations, are the most interest- 
ing of the boring Mollusca. They are largely in- 
habitants of the tropical waters, but have in the 
timber of ships been introduced almost everywhere 
over the wide sea. The body proper of this 
singular creature is usually very small, but through 
the great elongation backward of the siphons the 
animal is made to appear worm-like (although it 
has no near relation to the worms), and to attain a 
length varying in the different species from one 
to three or even six feet. The shell is compact, 
ridged, and open at both ends, and only sufficiently 
large to cover the anterior or body-portion, the 
animal thus appearing naked. Beginning in very 
early life, when frequently not over two weeks old, 
and when only of about the size of the head of a pin, 
the young ship-worm or Teredo puts itself to the 
task of boring, selecting for its base of operations 
all forms of wood or timber that may be immersed 
in water, whether the belongings of ships, dikes, 
piles, or piers; indeed, one or more forms even 
