Coptolithus virgatus Black, 1973
Elliptical placoliths are composed of single-tiered distal and proximal shields linked by narrow double-tube cycles. The central area is broad and occupied by a conspicuous central complex often formed by grill-like structures, parallel-sided rays, or multi-segmented plates. A locking cycle occurs on the proximal side between the proximal shield and the central-area structures.
Under cross-polarised light in plan view, Coptolithus displays white interference colours, inclined extinction lines, and length-fast (–) elongation. The locking cycle remains in constant extinction, forming a dim band between the tube cycle and the shield.
Placolith coccoliths with distal and proximal shields closely resembling those of Watznaueria, including narrow tube cycles and well-developed shields. Distinguished from Watznaueria by the presence of a central area occupied by grill-like structures, parallel-sided rays, or multi-segmented plates. The central-area elements always exceed the width of the tube cycles and form a conspicuous structure in the centre of the coccolith. In contrast, Watznaueria possesses a simple central plate that may be obscured by broad tube cycles, as in the Watznaueria arata group, or associated with narrow tube cycles enclosing an open central area, as in the Watznaueria ovata group. Where present, the central plate in Watznaueria does not exceed the width of the tube cycles.
Black, M. 1973. British Lower Cretaceous Coccoliths. I-Gault Clay (Part 2). Palaeontographical Society Monograph 127: 49-112.