Thanks Medusa. I did a little bit more research as well. There's not much, but I found this description:
"a cross between an amphitheatre and a theatre, commonly found in Gaul. . 40 metres in diameter, it has a circular stone wall, surrounded by an earth viewing bank which does not completely encircle the arena."
This description would seem to more closely fit the name, but most references, like the ones you referred to, seem to be for a complete viewing circle, rather than the more usual () shape (like two theatres built back-to-back - bit like an oval - does that shape have a name in geometry? Just looked it up on Wikipedia, and yes it does it's a 'Vesica Piscis'. How educated am I?)
I digress.