The Collaborative International Dictionary of English v.0.48:

Thallous \Thal"lous\, a. (Chem.)
   Of or pertaining to thallium; derived from, or containing,
   thallium; specifically, designating those compounds in which
   the element has a lower valence as contrasted with the
   thallic compounds. [Written also thallious.]
   [1913 Webster]