Description: Narrow, straight-sided species of Triquetrorhabdulus with c-axis perpendicular to length of nannolith.
Remarks: Similar to T. carinatus in shape but with c-axis perpendicular to length of nannolith. Often lie in an orientation in which they show birefringence, so gypsum plate is useful toshow that they have c-axis perpendicular rather than parallel to nannolith length. N.B. rioi is a corrected form of the name rioensis as originally proposed by Olafsson (1989).
Range: NN6-7.
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