[scarmig]

The point about the most recent common ancestor is well taken, but, at least according to my layman's level understanding of ring species, the two ends of the "ring" aren't expected to interbreed. Wikipedia supports that definition (and if it's wrong, should be updated by someone more knowledgeable than myself).

[thaumasiotes]

You seem to be right about the idea of "ring species". Papers from the past several decades all introduce the concept the same way.

[scarmig]

Yeah, though I wouldn't be surprised if your suggested "continuous" ring species exists somewhere, with gene flow possible from both sides at every point but not from one side of the ring to the other (even if brought into the same geographic proximity).

In principle you even could have some kind of genetic helix, where one end of the ring species not only meets but overlaps with the other, perhaps going so far as to do a full wraparound a number of times. But probably the two ends of the ring would occupy too similar an ecological niche (even with all the genetic divergence) to allow for much time of overlap.