The Collaborative International Dictionary of English v.0.48:

archeopteryx \archeopteryx\ n.
   1. an extinct primitive toothed bird (Archaeopteryx
      lithographica) of the Upper Jurassic having a long
      feathered tail and hollow bones; usually considered the
      most primitive of all known birds. Same as
      archaeopteryx.

   Syn: archaeopteryx.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Archaeopteryx lithographica
    n 1: extinct primitive toothed bird of the Jurassic period
         having a long feathered tail and hollow bones; usually
         considered the most primitive of all birds [syn:
         archaeopteryx, archeopteryx, Archaeopteryx
         lithographica]