A group of four carnelian intaglio brooches

A group of four carnelian intaglio brooches, the brooches each set with a modern intaglio depicting one of the labours of Hercules: slaying the Learnaean Hydra, capturing the Ceryneian hind, capturing the Cretan bull, and stealing the mares of Diomedes, after neoclassical originals from the collection of Prince Stanislas Poniatowski, each mounted in gilt metal, one applied with black enamel, largest brooch 5.5 x 5.0cm