Hyalosphenia platystoma West, 1903

Diagnosis: Shell transparent, wide ovoid in broad view, barely longer than wide, and slightly narrowing towards the pseudostome. In profile, elongated elliptical with a notched pseudostome. Pseudostome linear, nearly as wide as the shell.

Dimensions: According to Cash et al. (1909): Shell length 40–42 μm; width 28–32 μm; aperture 28–32 μm.

Habitat: Very rare, between sediment of oligotrophic and mesotrophic water types.

Geographical distribution: UK, Austria, Belgium, France, Argentina.