  It is known that soliton solutions of the KP-hierarchy corresponds to
singular rational curves with only ordinary double points. In this paper we
study the degeneration of theta function solutions corresponding to certain
trigonal curves. We show that, when the curves degenerate to singular rational
curves with only ordinary triple points, the solutions tend to some
intermediate solutions between solitons and rational solutions. They are
considered as cerain limits of solitons. The Sato Grassmannian is extensively
used here to study the degeneration of solutions, since it directly connects
solutions of the KP-hierarchy to the defining equations of algebraic curves.We
define a class of solutions in the Wronskian form which contains soliton
solutions as a subclass and prove that, using the Sato Grassmannian, the
degenerate trigonal solutions are connected to those solutions by certain gauge
transformations
