Heterostroma nereidiis gen. et sp. nov. (Rhodophyta), a dorsiventral rhodomelaceous marine alga from Western Australia

Abstract Heterostroma nereidiis gen. et sp. nov. (Rhodomelaceae, Ceramiales) is described from Yanchep (type locality) and the Abrolhos Islands in Western Australia. The bases of the plants are prostrate, laminar, and anchored by cables of consolidated rhizoids. Decumbent to erect, linear, deeply serrate blades arise from marginal initals along the bases. The vegetative fronds are 1–4 times pinnate, with a lateral branch primordium borne on every axial tier in a ¼ spiral arrangement. Dorsal and ventral branch primordia are generally dormant, only the distichous lateral primordia growing out into determinate or indeterminate axes. Four uncorticated pericentral cells are formed, two lying dorsally and two ventrally. Ventral pericentral cells divide longitudinally, and the derivatives along adjacent axes form secondary pit connections, giving rise to membranous thalli. Dorsal pericentral cells divide transversely, obliquely and longitudinally, and cover the ventral thallus layer, three or four derivatives co...