A coordinate system which is similar to bispherical coordinates but having fourth-degree surfaces instead of second-degree surfaces for constant μ. The coordinates are given by the transformation equations x | = | a/Λ cn μ dn μ sn ν cn ν cos ψ y | = | a/Λ cn μ dn μ sn ν cn ν sin ψ z | = | a/Λ sn μ dn ν, where Λ congruent 1 - dn^2 μ sn^2 ν,