1979 Edition


  • William A. S. Sarjeant
Reference work entry
Acritarchs are microorganisms of uncertain affinity, having a hollow shell of highly varied shape (spherical, ellipsoidal, discoidal, elongate, or polygonal) composed of an organic substance or substances. Their size is between 7 and 1000 μm, most often less than 150 μm. The shell may be pitted, granular, or entirely smooth; it may be unornamented or may bear spines or other processes, raised ridges ( crests), pits, or granules. The distribution of the ornament may be quite random or it may show a consistent positional relationship—e.g., confinement to, or arrangement around, the poles of an ellipsoidal shell. Sometimes the shell is multiple, consisting of two separate, roughly concentric membranes of comparable thickness; sometimes the main shell is partially or entirely surrounded by a much more tenuous, often incomplete outer membrane; sometimes the shell bears a median flange or wing. ( Fig. 1.)
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© Dowden, Hutchinson & Ross, Inc. 1979

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  • William A. S. Sarjeant

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