The spelling of "Decagynian" may seem confusing at first glance, but it follows the rules of English phonetics. In IPA transcription, it is pronounced /dɛkəˈdʒɪniən/. The "deca" prefix means "ten," and "gyn" refers to "female," indicating that this word relates to a group of ten female organisms. The "i" in the middle is an infix used to link the two parts of the word, and the final "an" denotes that it is an adjective. While the spelling may look daunting, a knowledge of phonetics can clarify its meaning.
Pert. to the order of plants decagynia, having ten pistils, or whose pistils have ten free styles.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.