  We construct a family of integrable equations of the form
$v_t=f(v,v_x,v_{xx},v_{xxx})$ such that $f$ is a transcendental function in
$v,v_x,v_{xx}$. This family is related to the Krichever-Novikov equation by a
differential substitution. Our construction of integrable equations and the
corresponding differential substitutions involves geometry of a family of genus
two curves and their Jacobians.
