The problem of plasma stability concerning quasi-flute perturbations in a toroidal magnetic trap with shear is investigated. An analysis is based on a small-oscillation equation for the components of the plasma-volume element displacement obtained from the ideal MHD equation system using only the common properties of differential operators in an arbitrary stream-coordinate system and the assumption of small scale for the perturbation components transverse to the magnetic field. It is shown that in the approximation of an incompressible plasma these equations are reduced to an ordinary differential equation of second order, which is transformed to the form of a stationary Schrodinger equation. The discrete eigenvalue spectrum of this equation describes unstable states of an arbitrary toroidal system with shear and the condition of absence of this spectrum coincides with the general geometric Mercier stability criterion. The dispersion relations relating quasi-flute instability growth rate to local plasma parameters are derived. The general geometric stability criterion is generalized to the case of finite ion Larmor radius.