Multiple plectrae

Hmmm... might have to dispute that. If I remember my Latin correctly, masculine singular nouns ending in um mutated to a in the plural. So:

singular plectrum becomes plural plectra

Whereas feminine singular nouns ending in a mutated to ae in the plural. Thus:

singular puella becomes plural puellae

Remember the graffiti scene in the "Life of Brian"? I was that Centurion... :-)