Definition of sea-purse:
(n) :
The egg case of skate, shark or ratfish; a mermaid's purse.
(n) :
The sea bean Dioclea reflexa.
(n) :
(rare) Alternative form of sea puss (“dangerous current”). [A strong seaward current; a riptide or undertow, especially as results when a sandbar formed by waves suddenly gives way, and which is dangerous to swimmers.]