Photograph by R. E. Halling, © 2004
This is a rare Russula in Costa Rica, originally described by Gertrude Burlingham from Florida. Since then, known distribution has been extended northward to New Jersey in the eastern United States. It has very crowded, narrow lamellae, whitish and hard basidiomata with involute pileus margins when young. Bart Buyck (Paris) kindly provided the determination and surmises that R. japonica may be the same thing.