The Collaborative International Dictionary of English v.0.48:

Bivium \Biv"i*um\, n. [L., a place with two ways. See
   Bivious.] (Zool.)
   One side of an echinoderm, including a pair of ambulacra, in
   distinction from the opposite side (trivium), which includes
   three ambulacra.
   [1913 Webster]