The Collaborative International Dictionary of English v.0.48:

Psalterium \Psal*te"ri*um\, n.; pl. Psalteria. [L., a
   psaltery.] (Anat.)
   (a) The third stomach of ruminants. See Manyplies.
   (b) The lyra of the brain.
       [1913 Webster]

WordNet (r) 3.0 (2006):

psalterium
    n 1: the third compartment of the stomach of a ruminant [syn:
         psalterium, omasum, third stomach]