Masses of new mesons and effects of the fifth flavour

Masses of new mesons are studied in terms of a mass formula which is a hybrid of the linear and quadratic form. A universal character of the formula which was observed in the baryon multiplets is examined in the meson multiplets. Effects of possible existence of the fifth flavour on the mass and mixing parameters of eta sub(c) meson (charmed-pair dominant pseudoscalar meson) are studied and it is found that the effects are significant if the mass of pseudoscalar meson with fifth flavour is less than 10 GeV. Some reasoning of hybrid mass formula and related problems are also discussed. (auth.)