Menuki with various shells (貝).

Menuki Omote: 30.5mm length, 14mm width, 5mm height

Menuki Ura: 33mm length, 11.5mm width, 5mm height

These menuki represent a conch, a scallop and an abalone.
This is a well-known motif since the Heian period because at that time the Japanese nobility had a game called 'Kai-Awase' (貝合わせ), consisting of combining pairs of shells with their corresponding half.
On a kimono this design refers to the marital harmony; it is also a symbol of cultural elegance, shells recalling the refinement and sophistication of the Heian period. The age of the game, played for hundreds of years gives a sense of longevity and continuity, symbol of traditions that persist.
Japan being an island, shells were widely used as food but also as coins and materials (mother of pearl). Conch shells were particularly used in wind instruments, in Buddhist rituals and shugendo to scare away evil spirits but also in combat to lead troops.

Shells also have a symbolism of protection, eternity, fertility and abundance.