  We introduce a new integrable hierarchy of nonlinear differential-difference
equations which is a subhierarchy of the 2D Toda lattice defined by imposing a
constraint to the Lax operators of the latter. The 2D Toda lattice with the
constraint can be regarded as a discretization of the BKP hierarchy. We
construct its algebraic-geometrical solutions in terms of Riemann and Prym
theta-functions.
