Founding of Colonies

The Roman concept of the day: sulcus primigenius or "initial furrow".

A definition via Sextus Pompeius Festus, a grammarian, from the ~2nd century CE.

“Primigenius sulcus dicitur, qui in condenda nova urbe tauro et vacca designationis causa inprimitur.” primigenius sulcus is so called: when a new city is being founded, it is impressed [or traced] with a bull and a cow for the purpose of demarcation.”-Festus, De verborum significatione (Paulus Diaconius Excerpta ex libris Pompei Festi de verborum significatu p142 285)

Plutarch describes the founding of Rome:

"And the founder, having shod a plough with a brazen ploughshare, and having yoked to it a bull and a cow, himself drove a deep furrow round the boundary lines, while those who followed after him had to turn the clods, which the plough threw up, inwards towards the city, and suffer no clod to lie turned outwards. With this line they mark out the course of the wall, and it is called, by contraction, “pomerium,” that is, “post murum,” behind or next the wall. And where they purposed to put in a gate, there they took the share out of the ground, lifted the plough over, and left a vacant space."
-Plutarch, Life of Romulus, 11

Servius, a 4th-century Latin grammarian and commentator on Virgil, extensively quoted Cato the Elder because his historical work, Origines, was a crucial and relatively rare source of early Roman history. The passage in Virgil describes the establishment of the Trojan colony of Acesta in Sicily.

Les origines : (fragments) by Cato, Marcus Porcius, 234 B.C.-149 B.C. (Servius, ad Verg. Aen. 5.755 quoting Cato, Origines)

“He marks out the city with a plough - a practice which Cato, in his Origines, says was the custom. For when founding a community, they yoked a bull on the right (outer) side and a cow on the inner side; and, girded in the Gabine fashion - that is, with part of the toga covering the head and part tucked up—they held a curved plough‑handle so that all the clods would fall inward; and thus, by drawing a furrow, they marked out the line of the walls, lifting the plough at the places where the gates would be.”
-Servius, ad Verg. Aen. 5.755 (quoting Cato, Origines) 

Les origines : (fragments) by Cato, Marcus Porcius, 234 B.C.-149 B.C. (Servius, ad Verg. Aen. 5.755 quoting Cato, Origines)

“Whoever is going to found a new city, let him plough with a bull and a cow; where he has ploughed, let him build a wall; where he wants there to be a gate, let him lift up and carry the plough, and call that place the gate.”
-Servius, ad Verg. Aen. 5.755 (quoting Cato, Origines) 

The scene on my latest coin of Augustus shows the sulcus primigenius, the ritual “first furrow” plowed with a yoke of oxen to mark the pomerium of a newly founded Roman colony - a stock motif on colonial coinages. Ancient authors connect the rite with Rome’s own foundation; colonial issues use it to proclaim legal status and re-foundation.  Although it isn't exactly a rare coin (150 specimens in RPC) it is a particularly nice example:

Macedonia,Alexandria, Troas (Conventus of Cyzicus), Octavian as Augustus, 27 BC – 14 AD Bronze circa 27 BC - 14 AD, AE (18.9 mm, 3.80g). Obv: AVG Bare head right Rev: Two priests plowing right (sulcus primigenius) Ref: RPC 1656. SNG Cop. 282 (older attribution).

Dated broadly to Augustus’ reign, this local bronze belongs to the wave of Augustan colonial re-organizations and veteran settlements that followed the civil wars; the simple AVG legend and bare head are entirely in keeping with provincial bronzes struck under or for the new regime.

Alexandria Troas, a major Aegean port in the Troad, became a Roman colony under Augustus (styled Colonia Alexandria Augusta Troas). A foundation‑plough reverse is thus programmatic for a city asserting its colonial status in the new Augustan order. (An older “Philippi” attribution before Burnett-Martin re-attributed in 2017 to Alexandria,Troas, likely reflected the same logic, since Philippi too was an Augustan colony. RPC now assigns this specific type to Alexandria Troas.)

References

• Katsari–Mitchell, The Roman Colonies of Greece and Asia Minor (2008) [PDF]

• A. Burnett & K. Martin, “An Early Imperial Coinage from Alexandria Troas?” in O. Tekin (ed.), Second International Congress on the History of Money and Numismatics in the Mediterranean World, 5–8 January 2017, Antalya: Proceedings (Istanbul: AKMED / Koç University Suna & İnan Kıraç Research Center for Mediterranean Civilizations, 2018), pp. 245–252. ISBN 978‑605‑211‑669‑2. 

• Smith, Christopher. 2017. “Servius, Cato the Elder and Virgil.” Mélanges de l’École française de Rome – Antiquité 129 (1): 85–100. https://doi.org/10.4000/mefra.4151