Sea Song - Kaiuta

...Note that shells with poems written on them are generally called utagai today. In the Muromachi period, such as in the Nobutanekyoki, it is said that shells with poems or poems written on them were called kaiuta and were used for calligraphy and as gifts, but in documents from the Edo period, utagai are recorded separately as utagaruta. Historically, it is easy to assume that shells with poems written on them later transitioned into utagaruta, but only shells with pictures drawn on them remain today, and no shells with poems written on them remain. ...

*Some of the terminology that refers to "Kaiuta" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…なお，現在，歌を書いた貝を一般に歌貝と呼んでいる。室町時代の《宣胤卿記》などに，貝の裏に歌や詩を書いて貝歌と呼び，揮毫（きごう），贈答に用いたとあるが，江戸時代の文献には歌貝は歌がるたとして別に記している。歴史的には詩歌が書かれた貝覆の貝が，のちに歌がるたに移行していったことは十分に推測できるものの，現存するものは絵を描いた貝だけで，詩歌を書いた貝は残されていない。…

※「貝歌」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報