Rihanna slams Amanda Bynes over Chris Brown tweet

Run down actress Amanda Bynes just inexplicably railed on Rihanna on Twitter, saying Chris Brown beat her because she's ugly ... but trust Rihanna not to go down without a fight.

Bynes tweeted -- and quickly deleted -- "@rihanna Chris Brown beat you because you're not pretty enough."

See the tweets after the cut....

and Riri replies: