>at least in Medieval Greek writing, the "C" is indeed used to represent "sigma". This occurs mid-word as well as in word-final position. I can't think of any word-initial examples. More commonly, ordinary sigma, in word-final, looks like a blend of an "s" and a "c"--a "c" with a little tail<

The confusion is due to the difference betwen written and printed Greek (ancient and medieval). Briefly, the lunate sigma, as it's called (the C-shaped one), is the more authentic form of the letter, although, for reasons that would be tedious to relate, 'authenticity' is not easy to define here. At any rate, it has nothing to do with the letter C. Texts that are "authentically" transcribed from inscriptions, papyri, and the like tend to use the lunate sigma in all positions; conventional printed Greek usually uses the c-with-tail-sigma in the final position and the closed sigma in others. But you will sometimes see the lunate sigma there too, as a nod to 'authenticity' and a move away from the conventional Greek typefaces, the most familiar of which, called 'Porsonian' faces, were based on the handwriting of the great eighteenth-century classicist Richard Porson-- a character Nabokov would have loved, by the way.

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