I'm responding from home--w/o rfc books--but I really think that there's a
strong tendency for 3d-declension dental stems with preceding iota to form
the accusative in -IN just like pure I-stems (e.g. POLIS), and that the
acc. forms in -IDA or -ITA, although historically original and probably
enduring variants, were generally secondary forms. I don't think there's
any real difference in meaning--it's not quite like the semantic
differentiation taking place in Italian CAUSA and COSA that both derive
from Latin CAUSA.
Carl W. Conrad
Department of Classics, Washington University
One Brookings Drive, St. Louis, MO, USA 63130
(314) 935-4018
cwconrad@artsci.wustl.edu OR cwc@oui.com
WWW: http://www.artsci.wustl.edu/~cwconrad/