Biscutum testudinarium Black in Black & Barnes, 1959
Cenorbiculus Burns, 1976
Small circular placoliths have two shields, which may be equal in size or with the distal shield larger. The tube cycle may be distinct, indistinct, or absent, and is always without a distal process. The central area is rarely open.
In plan view under XPL, the distal shield is in constant extinction because the optic axis (c-axis) is perpendicular to the microscope stage. The proximal shield often displays a white interference colour and exhibits length-fast (–) elongation (as in Biscutum centriperforatus), but it may rarely show constant extinction when the optic axis is perpendicular to the microscope stage (as in Biscutum testudinarium). The tube cycle, if present, displays a white interference colour and exhibits length-fast (–) elongation (as in Biscutum testudinarium).
Three genera—Bidiscus, Biscutum, Discorhabdus, and Palaeopontosphaera—are very similar and are often confused. In this work, these genera are defined as follows, consistent with the holotypes of their type species:
Bidiscus (Type species: Bidiscus cruciatus cruciatus Bukry, 1969): Circular placolith with square distal ornamentation.
Biscutum (Type species: Biscutum testudinarium Black in Black & Barnes, 1959): Circular placolith lacking any distal process or ornamentation.
Discorhabdus (Type species: Rhabdolithus patulus Deflandre in Deflandre & Fert, 1954): Circular placolith with a permanent distal process.
Palaeopontosphaera (Type species: Palaeopontosphaera dubia Noël, 1965): Often elliptical placolith with a distinct tube cycle but without any distal process or ornamentation.
Black, M. & Barnes, B. 1959. The structure of Coccoliths from the English Chalk. Geological Magazine. 96(5): 321-328.
Bukry, D. 1969. Upper Cretaceous coccoliths from Texas and Europe. University of Kansas Paleontological Contributions, Articles 51 (Protista 2): 1-79.
Burns, D. A. 1976. Nannofossils from the Lower and Upper Cretaceous deposits, Nettleton, Lincolnshire, England. Revista Española de Micropaleontología. 8(2): 279-300.
Deflandre, G. & Fert, C. 1954. Observations on current and fossil coccolithophorids in ordinary and electron microscopy. Annales de Paléontologie 40: 115-176.
Noël, D. 1965. Sur les Coccolithes du Jurassique Européen et d'Afrique du Nord. Essai de classification des coccolithes fossiles. Éditions du Centre National de la Recherche Scientifique, Paris. 1-129.