Another way in which humans can, and have, produced holes in
the umbo region of bivalve shells is an incidental byproduct of
opening the shells to extract the meat.  Anadara (tuberculosa,
similis, and grandis) valves are not infrequently found in Ecuadorian
shell middens with largish holes (about 5-15 mm in diameter) at the
umbo. These shells, especially the thick and heavy valves of A.
grandis, are often assumed by archaeologists to be net weights,
and perhaps they were actually used as such.  These species
were opened by placing them umbo down on a hard surface,
inserting a blade between the valves, and striking the blade with a
hammer stone.  Anadara species are still opened in this way,
though now with a steel blade.  Occasionally, the shell will slip to
the side as the blade is struck and the force will be concentrated
on the side of the umbo.  When this happens, a nearly perfect
circular piece of shell will break out, somewhat similar to the way a
beebee striking plate glass will break out a conical piece. The
thicker valves seem to produce more circular holes.  I have
accidentally had the same thing happen by dropping Anadara onto
a cement floor.

As for producing beads by grinding away the spire of gastropod
shells, such beads are common and can often be distinguished
from beachworn shells because only the spire is worn and, if the
shell artifact is in good condition, it can be seen with a hand lens
that the abrasions are all in one direction.

kate