From Art to Artifact:  Making Sense of Roman Coins

The nine types which feature the muses bear a mark on the obverse in the field behind Apollo’s head. Shown here is the sandal which appears on the Thalia type. These symbols served as control marks used to identify and seriate the coins. On Musa’s coins the marks are also paired with the reverse types as an additional way of identifying the muse. This is demonstrated by clear correllations between the mark and the muse’s attributes (cf. Reverse Types of the Muses):
Calliope with lyre key
Clio with scroll
Erato with flower
Euterpe with flutes
Melpomene with sceptre
Polymnia with wreath
Terpsichore with tortoise / flower
Thalia with Sandal
Urania with Star