Cortical shell ("twin shell") cylindrical to ellipsoidal with a very conspicuous equatorial constriction. Two medullary shells (outermost slightly compressed, innermost spherical) joined to the cortical by several radial beams confined to the equatorial plane (Didymocyrtis tetrathalamus 2) May have polar caps (D. tetrathalamus intro) or spines, and supplemental peripheric growth (mantle; Didymocyrtis tetrathalamus). Height of cortical twin shell: 90-140 µm, height of polar caps: 36-63 µm.
The subspecies Dydimocyrtis tetrathalamus coronatus is identical to Dydimocyrtis tetrathalamus tetrathalamus, except for the development of a corona of stout, unbranched, 3-bladed spines on the distal ends of the twin shell and/or on the polar caps (Nigrini, 1970).
Ref.: Nigrini and Moore (1979).