  A generalized hyperbolic tetrahedra is a polyhedron (possibly non-compact)
with finite volume in hyperbolic space, obtained from a tetrahedron by the
polar truncation at the vertices lying outside the space. In this paper it is
proved that a volume formula for ordinary hyperbolic tetrahedra devised by J.
Murakami and M. Yano can be applied to such ones. There are two key tools for
the proof; one is so-called Schlafli's differential formula for hyperbolic
polyhedra, and the other is a necessary and sufficient condition for given
numbers to be the dihedral angles of a generalized hyperbolic simplex with
respect to their dihedral angles.
