(Perhaps in the interests of having something one can actually dance -- and walk -- in, this description of a mermaid costume blithely ignores the fish-tail aspect!)
Dress with low bodice of eau de Nile silk, covered with drapery of sea green tulle, with a profusion of white corals, shells, marine grass, flowers, and crystallised foam; the left shoulder of the dress ornamented with a cluster of diamonds; the right shoulder and ceinture with silvery iridescent gems; flowing hair crowned with corals, pearls, and diamonds; interspersed with pendants of seagrass.
Holt, Ardern. Fancy Dresses Described, 5th Edition. London: Debenham & Freebody, 1887.