I was informed as follows: This relief slab of the sarcophagus represents the goat Amalthea nursing the infant Zeus and to the right Mercury with the caduceus.

From the Wikipedia: cadeceus: ()from Greek κηρύκειον kērukeion "herald's staff") is the staff carried by Hermes in Greek mythology. The same staff was also borne by heralds in general, for example by Iris, the messenger of Hera. It is a short staff entwined by two serpents, sometimes surmounted by wings. In Roman iconography it was often depicted being carried in the left hand of Mercury, the messenger of the gods, guide of the dead and protector of merchants, shepherds, gamblers, liars and thieves.