XanthidiumXan*thid"i*um\, n.; pl. Xanthidia. [NL., fr. Gr. xanqo`s yellow.] (Bot.) A genus of minute unicellular alg[ae] of the desmids. These alg[ae] have a rounded shape and are armed with glochidiate or branched aculei. Several species occur in ditches, and others are found fossil in flint or hornstone.