a mattress made of foamed plastic material with modulated rigidity , having an upper layer made of a plastic material with a composition based on an open cell polyurethanic foam with silicone , flameproofing , fungicide and bacteriostatic additives , having low elastic memory and being substantially inelastic , which is arranged on a lower layer which is composed of modular elements which are mutually assembled by joints of the mortise and tenon types .