We don't have synonyms for phellum yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are phellem and pellum.
°(biology taxonomy) A rank in the classification of organisms, below kingdom and above class; also called a division, especially in describing plants; a taxon at that rank.
°(biology taxonomy) A rank in the classification of organisms, below kingdom and above class; also called a division, especially in describing plants; a taxon at that rank.