Petalophyllum is a simple thalloid liverwort that has been placed in the family Fossombroniaceae along with three other genera, Austrofossombronia, Fossombronia and Sewardiella. In 1844, Lehmann named the genus to include P. preissii from the Swan River in Australia. Since its naming, there have been six other species attributed to the genus. With the exception of a few well-studied localities in the British Isles, Petalophyllum is poorly known. It has been accepted that this is an important genus in terms of evolution within liverworts, but its actually position within the Metzgeriidae is debatable. This study was undertaken to define species limits and variation potential, determine evolutionary trends in shoot/sporophyte associations and sporophyte anatomy and describe biogeographical patterns within the genus. Loans were obtained from herbaria worldwide and lectotype specimens were designated. Morphological characters were studied using scanning electron and optical microscopes and paraffin-sectioning techniques. Consequently, previous segregation of Petalophyllum along with Sewardiella into a separate subfamily, the Petalophylloideae, has been re-evaluated. Of the seven original Petalophyllum species named, only P. preissii in New Zealand and Australia, P. indicum in India and Pakistan and P. ralfsii in Europe and Algeria are maintained. P. bolivianum, named from Bolivia, was determined to be Gongylanthus.

Key words: anatomy, liverworts, Metzgeriidae, monograph, Petalophyllum