GENUS AND SPECIES CONCEPTS IN ZONARIA AND HOMOEOSTRICHUS (DICTYOTALES, PHAEOPHYCEAE), INCLUDING THE DESCRIPTION OF EXALLOSORUS GEN. NOV

Examination of type material, comparative studies on vegetative charac ters and data from published accounts of sporangia, gametangia and spe rmatozoids are used here to further develop and strengthen generic and species concepts in Zonaria and Homoeostrichus. Zonaria is characteri zed by octosporangia which lack a stalk cell and occur among whitish p araphyses, by the presence of cortical cells in indusiate sporangial a nd oogonial sori, and by slightly elongate sterile cells bordering the antheridial sori. Other characters (mucilage in sporangial and oogoni al sori, biflagellate sperm with a row of many tall spines along the a nterior flagellum) may also be diagnostic of Zonaria but have not been demonstrated in the generitype. Eight species are retained in Zonaria . Generic placement of Zonaria stipitata, described on vegetative char acters, requires confirmation. Homoeostrichus, as typified by Homoeost richus sinclairii, is defined by the presence of brown paraphyses in n onindusiate sporangial and oogonial sori and bordering antheridial sor i, a multicellular sporangial stalk, oogonia without a stalk cell, and uniflagellate spermatozoids. Zonaria harveyana and Homoeostrichus ols enii are similar in sporangial structure, but they cannot be assigned to either Zonaria or Homoeostrichus as now defined and consequently ar e transferred to the new genus, Exallosorus, as Exallosorus harveyanus (Pappe ex Kutzing) comb. nov. and Exallosorus olsenii (Womersley) com b. nov. Exallosorus is distinguished from Zonaria and Homoeostrichus u sing sporangial characters: tetrasporangia have a stalk cell and are c losely packed into indusiate sori which lack paraphyses and mucilage. Problems in obtaining reliable species identification for the five sou thern Australian species of Zonaria are partially resolved. This study demonstrates again the essential contribution of reproductive charact ers to the definition of taxa and supports the position that decisions to erect, subsume or transfer members of the Dictyotales should be ba sed on more than vegetative characters.