Fractional differential equations have attracted considerable interest because of their ability to model many phenomena, due to anomalous diffusion effects in constrained environments [1].A physical-mathematical approach to anomalous diffusion may be based on generalized partial differential equations containing derivatives of fractional order [2].Recently a model has been proposed for describing processes that become less anomalous as time progresses by the inclusion of a secondary fractional time derivative acting on a diffusion operator [3].