Pervilithus varius Crux, 1981
In plan view, under crossed polars, Pervilithus displays first-order reddish-yellow interference colours and inclined extinction lines with length-fast (-) elongation.
Species of the family Lapideacassaceae cannot be reliably identified in plan view and are therefore assigned to Pervilithus, which exhibits a narrow wall composed of 7–12 segments surrounding a hollow central area. Specimens assigned to Pervilithus require examination in a mobile mount to determine whether the number of segments is diagnostic at the species level.
Crux, J. A., 1981. New calcareous nannofossil taxa from the Cretaceous of South East England. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 1981, 633-640.