The Japanese name Sode-gai usually is used for some species of the gastropod
family Strombidae (currently Sode-bora is used for the family name). Sode means
"sleeve" and you may easily understand why this term is used for the Strombidae.
Strombus vittatus japonicus (in Japanese books as Doxander japonicus) was used
to be called specifically as "Sode-gai" as well as its another name "Shidoro".

I am not sure why any of the pearl oysters or unionids was called as "Sode-gai"
by Von Siebold. In my reference collection I examined, there is no indication of
such a local name for any pearl oysters or freshwater mussels in Japan.

Because I am not familiar with unionids, you may ask the expert at Lake Biwa
Museum: [log in to unmask] who may be able to shed a light on this problem.

Masashi Yamaguchi
e-mail: [log in to unmask]
HTTP://www2s.biglobe.ne.jp/~big/coral/
Department of Marine Sciences,
University of the Ryukyus
1 Senbaru, Nishihara, Okinawa 903-0213 Japan