bivium

from The Century Dictionary.

• noun In echinoderms, the ambulacra of the two posterior arms or rays taken together and distinguished from the three anterior rays collectively. See trivium, and cut under Spatangoida.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) One side of an echinoderm, including a pair of ambulacra, in distinction from the opposite side (trivium), which includes three ambulacra.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology One side of an echinoderm, including a pair of ambulacra, in distinction from the opposite side (trivium), which includes three ambulacra.