• Category
  • Length Range
    16-30 µm
  • Width Range
    6–9 µm
  • Striae in 10 µm



Valves are elliptical-lanceolate with rostrate to capitate apices, 6-9 µm wide and 16-30 µm long. The raphe is straight and filiform. Externally, the proximal raphe ends are surrounded by two small grooves. The axial area is narrow and linear. The central area is rectangular to irregular in shape, with an isolated stigma. Striae are uniseriate and radiate, becoming parallel or slightly convergent at the ends. The striae are also usually deflected, or bent, and number 14-18 in 10 µm. Short and long striae alternate in the central valve. Areolae are apically elongated and number approximately 60-70 in 10 µm. Subpolar areolae (annulae) are larger than the other areolae, forming two rows on either side of the raphe at the poles. Internally, the annulae are surrounded by spherical nodules, typical of the genus Geissleria.


Fresh waters of moderate to high ionic strength

Original Description

Long. 16-22 µ, Lat.6-7 µ. Str. 16-20 in 10 µ, transverse lineatis. Valva elliptica, apicibus capitatis. Raphe directa, area hyalina angusta, media in parte valvae in aream transersam dilatata, cincta. Striis radiantibus, in apicibus convergentibus, mediis tribus longiudine diversa itaqve inter se remotis, ut amplificatione mediocre figura decussis existat. Nodulis terminalibus summis in apicibus situatis.

  • Basionym
    Navicula decussis
  • Author
    Østrup 1910
  • Length Range
    16-22 µm
  • Width
    6–7 µm
  • Striae in 10µm

Original Images

Gedecussis  Orig Illus
Gedecussis  Orig Descr

Cite This Page

Potapova, M. (2009). Geissleria decussis. In Diatoms of North America. Retrieved April 16, 2024, from https://diatoms.org/species/geissleria_decussis


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Geissleria decussis from all the stream reaches where it was present. Note that the relative abundance scale is the same on each plot. Explanation of each environmental variable and units are as follows:

ELEVATION = stream reach elevation (meters)
STRAHLER = distribution plot of the Strahler Stream Order
SLOPE = stream reach gradient (degrees)
W1_HALL = an index that is a measure of streamside (riparian) human activity that ranges from 0 - 10, with a value of 0 indicating of minimal disturbance to a value of 10 indicating severe disturbance.
PHSTVL = pH measured in a sealed syringe sample (pH units)
log_COND = log concentration of specific conductivity (µS/cm)
log_PTL = log concentration of total phosphorus (µg/L)
log_NO3 = log concentration of nitrate (µeq/L)
log_DOC = log concentration of dissolved organic carbon (mg/L)
log_SIO2 = log concentration of silicon (mg/L)
log_NA = log concentration of sodium (µeq/L)
log_HCO3 = log concentration of the bicarbonate ion (µeq/L)
EMBED = percent of the stream substrate that is embedded by sand and fine sediment
log_TURBIDITY = log of turbidity, a measure of cloudiness of water, in nephelometric turbidity units (NTU).
DISTOT = an index of total human disturbance in the watershed that ranges from 1 - 100, with a value of 0 indicating of minimal disturbance to a value of 100 indicating severe disturbance.