The Collaborative International Dictionary of English v.0.48:

Episyllogism \Ep`i*syl"lo*gism\, n. [Pref. epi- + syllogism.]
   (Logic)
   A syllogism which assumes as one of its premises a
   proposition which was the conclusion of a preceding
   syllogism, called, in relation to this, the prosyllogism.
   [1913 Webster]