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Specimens of cyanobacteria were collected from three geographically distant caves in Greece, and an interesting species of the Oscillatoriales was isolated and analyzed. Morphological characters were analyzed from fresh and cultured material using LM, SEM and TEM; moreover, molecular analysis based on the 16 S rRNA sequence was applied. This cyanobacterium bears a characteristic blackish, stratified, mucilaginous sheath and it is characterized by entangled filaments, facultative presence of false branching, presence of necridia and peripheral arrangement of thylakoids. The autapomorpic blackish mucilaginous sheath and the pseudobranching filaments are unique morphological characters within the genus Phormidium, according to classical taxonomy. The 16 S rRNA sequence showed relatedness to Phormidium priestleyi ANT L52.6 with 97-98 % similarity, but without morphological support. Both phenotypic and genetic traits supported the designation of a new species, under the name Phormidium melanochroun Lamprinou & Pantazidou sp. nov.

key Ic'ords: ~~l o l~n s c a , V d t~~t a , shell ultrastructure, scannin~-clectron-microscopy, epizoic diatoms -4bstractThe irltrastructnre of Valucrfn pscsnalzs 0. F. M~L L E H . , a mollrisc collected from Lake Trichonis, Greece, is illustrated by SEN and compared with Polish specimens. The specif ic features observed are leading to the characterization of the specimens as an ecotype. The epizoic algal grot+ ths, ascertained by direct esaniination or scraped from shells of V . pzscinalas, are commented.