• Category
  • Length Range
    24-50 µm
  • Width Range
    13-18 µm
  • Striae in 10 µm



Valves are elliptic, with rostrate to rostrate/apiculate to capitate/apiculate apices. The axial area is narrow and expanded at the center to form a transapically oval to circular central area. The raphe is filiform, straight to slightly undulate. The external proximal raphe ends are prominent and dilated. Areolae are punctate. Striae are decussate, that is, areolae are arranged in multiple rows - a transapical row and two oblique rows.

This is a distinctive species. Our observations expand the size range of this taxon beyond that reported by Patrick and Reimer (1966). Note that the valve apices are variable in shape and typically more rostrate than the population illustrated here (Edlund et al. 2006).


Widely distributed, found commonly in aerophilous conditions

Original Description

  • Basionym
    Navicula placenta
  • Author
    Ehrenb. 1854

Original Images

Decussata placenta orig illus

Cite This Page

Kociolek, P. (2011). Decussata placenta. In Diatoms of North America. Retrieved July 15, 2024, from https://diatoms.org/species/decussata_placenta


The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Decussata placenta 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.