Abstract
A new species, Encyonopsis indonesica, is described from the ancient lake Matano, Sulawesi island, Indonesia. The morphology of this species was studied by means of light and scanning electron microscopy. E. indonesica has a remarkable valve ultrastructure. The valve surface is ornamented with numerous longitudinal siliceous ribs and siliceous verrucae. Valve face delineated from the mantle by a thickened marginal ridge. Raised sterna border the raphe branches. Raphe is distinctly undulate with distal ends hooked strongly to the ventral side. The only similar species to E. indonesica is Amphora dissimilis described from New Caledonia. Comparison of both taxa is given and A. dissimilis is transferred to Encyonopsis. The taxonomic placement of both taxa is evaluated, and the phenomenon of external siliceous ornamentation is discussed.