The Collaborative International Dictionary of English v.0.48:

Trilobite \Tri"lo*bite\ (tr[imac]"l[-o]*b[imac]t), n. [Cf. F.
   trilobite. See Trilobate.] (Paleon.)
   Any one of numerous species of extinct arthropods belonging
   to the order Trilobita. Trilobites were very common in the
   Silurian and Devonian periods, but became extinct at the
   close of the Paleozoic. So named from the three lobes usually
   seen on each segment.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trilobite
    n 1: an extinct arthropod that was abundant in Paleozoic times;
         had an exoskeleton divided into three parts