The Collaborative International Dictionary of English v.0.48:

Micraster \Mi*cras"ter\, n. [NL., fr. Gr. mikro`s small + ?
   star.] (Paleon.)
   A genus of sea urchins, similar to Spatangus, abounding in
   the chalk formation; -- from the starlike disposal of the
   ambulacral furrows.
   [1913 Webster]