Species of the Pylaisiadelphaceae (Musci) known from Japan up to the present were briefly reviewed with a special reference to Yakushimabryum longissimum, recently described from Yakushima Island, southern Kyushu. The species is characterized by more or less shinny, yellowish green color in plants in the fields, alar regions of leaves composed of quadrate cells arranged in scalariform manner, ascending branches often elongated (but not caudate) into slender tips, and filamentous gemmae with smooth, rectangular cells. Although these morphological features suggest its affinity to Clastobryopsis or Gammiella, molecular analysis using rbcL sequences indicates its phylogenetic position near to Isocladiella, Isopterygium or Peterogonidium. Keys to genera and species of the Japanese Pylaisiadelphaceae along with an exotic species of Clastobryopsis planula are provided with a brief note on the inter-relationship between C. robusta and C. planula.

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