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An updated circumscription is proposed for the genus Trematocarpus, including new data based on morphological and reproductive studies of the type species Trematocarpus dichotomus. The material studied included T. dichotomus, T. acicularis and T. concinnus from Pen, Chile, New Zealand and Australia. Bibliographic information on T. pygmaeus, T. fragilis, T. flabellatus, T. papenfussi and T. affinis is also given. The initial diagnosis permitted the inclusion solely of species having cylindrical thalli, height no greater than 5 cm, zoned tetrasporangia and protruding cystocarps, and was based on descriptions of individuals of T. dichotomus and T. virgatus. The latter species, mis-identified, proved to be Sarcodiotheca gaudichaudii, and thus the description of the genus was based on characteristics of two species from two different genera, later partially clarified by Schmitz (1989) who lectotypified the genus based on the characteristics of T. dichotomus from Peru. The present study completes and corrects the original diagnosis, extending maximum sizes to 30 cm, thalli cylindrical, subcylindrical or flattened in cross-section, localized origin of gametic and sporific reproductive organs on the concave or convex surface in flattened thalli, and over the entire surface of cylindrical or subcylindrical thalli, and adhesion by a disc with or without stoloniferous projections. New information is presented on the appearance and development of the carpogonial branch from a differentiated intercalar cell in the cortex, which develops a carpogonial filament of three or four cells through successive divisions. Processes of post-fertilization show procarpial fusion, fusion of gonimoblastic cells and external generation of cystocarps. It is also noted that the species are dioic. This description sets apart the genera Sarcodiotheca, Agardhiella and Eucheuma, which have morphological and reproductive characteristics similar to Trematocarpus, and correctly incorporates the eight currently recognized species of the genus. [ABSTRACT FROM AUTHOR]