rice-shell

from The Century Dictionary.

• noun A shell of the genus Olivella, of about the size and whiteness of a grain of rice: sometimes extended to similar shells of the family Olividæ. See cut under olive-shell.
• noun In Australia, any one of various species of Truncatella, a small marine mollusk. The shell is used for necklaces and derives its name from its resemblance to grains of rice.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) Any one of numerous species of small white polished marine shells of the genus Olivella.