Kofoid system

A convention, the “Kofoid System” of tabulation nomenclature (plate naming), is currently in universal use (see figure below). The plates in each latitudinal series are numbered from the cell’s left to right, beginning with the plate closest to the midventral position. It also uses a notation to designate the series, using primes to indicate the apical ('), precingular (''), postcingular ('''), and antapical ('''') plates, both when labelling plates on figures and when producing a plate formula. The latter is a listing of the total number of plates in each series for a species or genus. Cingulars (C), sulcals (S), anterior intercalaries (a), and posterior intercalaries (p) are designated by letters. The t plate is a small transitional plate between the cingulars and the sulcals at the proximal end of the girdle in peridinioids and at the distal end in gonyaulacoids. Other distinctions between peridinioids and gonyaulacoids include the common occurrence of 2–3a, 7'', 5''', 2'''' in the former and 6'', 6''', 1p, 1'''' in the latter (exceptions being due to apparent suture loss or plate subdivisions). Basic symmetry: the former tending to bilateral symmetry, the latter showing evident torsion.

Although the Kofoid System is usually easy to apply, ambiguities in the attribution of some plates to one series or another can cause problems. This, combined with the mechanical, consecutive numbering, renders the system poor for intergeneric comparisons. Taylor (1980) introduced an alternative model of plate nomenclature, elaborated on by Evitt (1985). It consists of three epithecal polar (A–C), six pre-equatorial (1–6), six equatorial (a–f), six postequatorial (I–VI), and three hypothecal polar (X–Z) sectors, which represent hypothetical primary plates from which homologous plates can be recognized by assuming subdivisions, suture losses, and plate size and position changes. The first step is to normalize the cell to a sphere, removing obvious plate distortions. Then the primary plates and their sutures are determined by studying the relationships of the plates to each other (see examples given by Evitt 1985).