The Collaborative International Dictionary of English v.0.48:

Xanthidium \Xan*thid"i*um\, n.; pl. Xanthidia. [NL., fr. Gr.
   xanqo`s yellow.] (Bot.)
   A genus of minute unicellular algae of the desmids. These
   algae 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.
   [1913 Webster]