Guide to Microfissurata

SPECIES OF MICROFISSURATA ARE NOT KNOWN TO OCCUR IN NORTH AMERICA

Species are comparatively small and usually live as solitary cells. Valves are lanceolate or linear elliptical, with protracted ends. The axial area is wide or very wide, especially in the central area. The central area is an expanded fascia. Externally, the striae are open slits, termed microfissures. Internally, the microfissures are closed and covered by hymenes that are slightly depressed within the alveoli. Areolae are absent, except for few subpolar striae divided by silica at the face/mantle junction. The external and internal proximal raphe fissures are slightly deflected to the primary side and an intermissio is present. The external distal raphe fissures are deflected to the secondary side. Internally, the distal raphe terminates in small, hemispheric, helictoglossae. One lobed plastid is present.

Microfissura differs from other small biraphid genera with chambered striae (Chamaepinnularia, Diadesmis, Gomphosphenia and Kobayasiella) by the presence of an intermissio and by the striae closed by internal, but not external, hymenes. In the light microscope, Microfissurata strongly resembles some Caloneis species. Note that Caloneis possesses areolae, however. A few species of Microfissura have been described, all from dystrophic habitats, such as springs and seepage pools, sites where Caloneis does not occur.