Melanothamnus harveyi is a small marine alga which grows in tufts no more than high. The erect branches are formed by a central axissurrounded by four perecentral cells of the same length. A cortex may develop by cells growing downwards in the grooves between the perecentrals. Latteral branches are usually dense. The holdfast is a disk of downgrowing filaments from which further erect branches develop. The general appearance is very variable.