Alisdair, Thanks so much for sorting my puzzle about the latin name Ebenus cretica. Your comments sent me back to my ancient and much neglected Kennedy's Latin Primer and my old school dictionary of classical latin.
Interestingly Arbutus, Laurus, Myrtus and Quercus also seem to be feminine nouns.
Somehow I have been thoroughly confused about the Ebenus in E.cretica and the ebony in ebony wood. The latter , it appears is usually a Diospyros (the same genus as persimmon). My small school dictionary defines ebenus as m., the ebony tree. That doesn't seem to be right in contemporary terms. I wonder how the name Ebenus attached itself to E. cretica. Is there something very hard about it? Also, it seems to have been an Onobrychis at some stage in its nomenclature.