Macrobius, Saturnalia 1.7.21:
“This Janus received Saturn, who was brought there by a ship, as a guest. When he had learned from him the art of agriculture and had made life better than it was before the discovery of fruit, he rewarded Saturn with a share in his kingship. Then, when he first stamped coins, he preserved this reverence for Saturn: on one side of the coin, his own head was pressed, and on the other – because Saturn had been carried by boat – there was a ship, which was meant to propagate the memory of Saturn to later generations. The fact that coins were thus decorated can be gleaned today from the expression which boys use when gambling; when they throw denarii into the air they yell ‘heads or ships,’ and this game serves as a testament to the ancient practice.”
Hic igitur Ianus, cum Saturnum classe pervectum excepisset hospitio et ab eo edoctus peritiam ruris ferum illum et rudem ante fruges cognitas victum in melius redegisset, regni eum societate muneravit. 22 Cum primus quoque aera signaret, servavit et in hoc Saturni reverentiam, ut, quoniam ille navi fuerat advectus, ex una quidem parte sui capitis effigies, ex altera vero navis exprimeretur, quo Saturni memoriam in posteros propagaret. Aes ita fuisse signatum hodieque intellegitur in aleae lusum, cum pueri denarios in sublime iactantes capita aut navia lusu teste vetustatis exclamant.