Source : Webster's Revised Unabridged Dictionary (1913)

Sea purse \Sea" purse`\ (s[=e]" p[^u]rs`). (Zo["o]l.)
   The horny egg case of a skate, and of certain sharks.

Source : WordNet®

sea purse
     n : the seaward undercurrent created after waves have broken on
         the shore [syn: {undertow}, {sea puss}, {sea-puss}, {sea-purse},
          {sea-poose}]