This paper thoroughly studies the specimens of two biraphid diatom species reported from China for the first time ——Prestauroneis tumida and Placoneis anglophila var. signata, which were collected from the East Dongting Lake National Nature Reserve, were studied with light and scanning electric microscopy. The results show that: (1) Pr. tumida possesses four identifying characters: ① valve outline lanceolate in larger specimens, and ellipticlanceolate in smaller specimens; ② striae radiate on the middle valve part and spaced wider than on other parts; ③ a pseudoseptum occurring at both apices; ④ the inner areola openings covered by hymens. (2) Pl. anglophila var. signata also possesses four identifying characters: ① linearelliptic or elliptic valve outline; ② striae radiate in most middle part where stria density is 10-14/10 μm, except at the apices where striae are parallel; ③ a stigma present at the center of the valve; ④ the internal areola opening covered by a velum which is supported by four props and the areola density is 35-40/10 μm. The detailed descriptions and clear illustrations in this paper provide a basis for the correct identification of Pr. tumida and Pl. anglophila var. signata for the future researchers.