In a way, it is not a
countermark, with the
types below "erased," and an
overstrike of a crab. I think if you look at catalogues, you'll find it listed under
Akragas, a lean period _after_
Akragas got defeated by somebody, Carthaginians, if I recall correctly, and I admit I might not be remembering it correctly, but that is enough information to look into it, if you care to do so.
And yes, in a way, it is a
countermark, but not functioning in the way of most
countermarks. Again, the
types of the original coin are obliterated, which is not the case for most
countermarked coins.