At the centre of the figural group above a rocky grotto stands Poseidon in a shell-shaped chariot, his trident in his hand. To his left is a nymph, while on his right kneels the sea-goddess Thetis, entreating Poseidon to favour the voyage of her son, Achilles, who has set off to conquer Troy. Frolicking at the foot of the grotto are the Tritons, creatures who are half-man and half-fish, and belong to Poseidon's entourage. They hold conch shell trumpets with which they can inspire fear in both man and beast, and are restraining the hippocampi or sea-horses who draw Poseidon's chariot across the seas.