As a rule, if there is an i before the -a, -us, -um in the original
genus, then the family name ought to end with iidae-you just keep the
original i, same as any other letter before the ending.  I know
offhand of one exception in Mollusca, where the Trapezidae was
deliberately spelled with only one i by a ruling of the ICZN to
resolve homonymy with an arthropod family.

--
Dr. David Campbell
The Paleontological Research Institution
1259 Trumansburg Road
Ithaca NY 14850

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