The Collaborative International Dictionary of English v.0.48:

Earthstar \Earth"star`\, n. (Bot.)
   A curious fungus of the genus Geaster, in which the outer
   coating splits into the shape of a star, and the inner one
   forms a ball containing the dustlike spores.
   [1913 Webster]

WordNet (r) 3.0 (2006):

earthstar
    n 1: any fungus of the family Geastraceae; in form suggesting a
         puffball whose outer peridium splits into the shape of a
         star