The Collaborative International Dictionary of English v.0.48:

Selenious \Se*le"ni*ous\, a. [Cf. F. s['e]l['e]nieux.] (Chem.)
   Of, pertaining to, or containing, selenium; specifically,
   designating those compounds in which the element has a lower
   valence as contrasted with selenic compounds.
   [1913 Webster]