It appears that, strictly, the plural form of 'polis' is 'poleis'. The iota-sigma ending is replaced with an eta-sigma ending in the plural.

    polis:

    poleis:
Methinks you mean the plural is ``poles'', not ``poleis''.

So it's pluralized like ``crises'', not like ``irides'', as I (admittedly) guessed.
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Bitt Faulk