The Collaborative International Dictionary of English v.0.48:

Protosulphide \Pro`to*sul"phide\, n. [Proto- + sulphide.]
   (Chem.)
   That one of a series of sulphides of any element which has
   the lowest proportion of sulphur; a sulphide with but one
   atom of sulphur in the molecule.
   [1913 Webster]