MetaMortician wrote:Thank you both. I actually like Arca Arcana. But I think Arca Arcanorum fits better with the meaning I wanted. However -arum wold be feminine. Is it ok to use -orum with feminine nouns???
No, it's not ok: the genitive plural of nouns of the first declension is arum. However, arcanum is 2nd declension neutral not feminine, hence its genive ends in orum, not arum.
Choose either arca arcana
or arca arcanorum
They're both grammatically correct, although they might raise a few classicists' eyebrows.
In the first case, arcana
is an adjective from arcanus,a, um agreeing with arca (1st declension, feminine, singular). In the second case, arcanorum
is a noun from arcanum, i (neutral, 2nd declension) in the genitive plural. Everything clear?