In this representation, Mercury is portrayed with another of his attributes: the tortoise (shown here between his feet). This animal is a direct reference to a mythological episode.
One day Mercury found a tortoise; the highly inventive deity decided to hollow it out and stretch ox gut strings across the shell cavity... and that was how he invented the lyre. Apollo, delighted by the instrument, exchanged his flocks for it. The tortoise is therefore an indirect symbol of Mercury’s cleverness.