Studies on the Genus Orbitolina (Foraminiferida)

The genus Orbitolina is described in detail and is shown to be represented by one species only: Orbitolina lenticularis (Blumenbach). This species can be subdivided into form-groups, based on the characteristics of the megalospheric embryonic apparatus. The evolution of the species is orthogenetic. The specimens probably lived with the apex of the cone pointed downward. The microspheric test starts with a strepto-spiral, the megalospheric test with an embryonic apparatus consisting of a proloculus, a deuteroconch, and a varying number of epiembryonic chambers. The embryonic apparatus is the only consistent feature on which the age of Orbitolina can be determined; the method applied will be described. The neanic chamber layers consist of tubular chamber passages; the chamber layers are interconnected by oblique, aligned stolons, placed alternately left and right of the chamber passages. The contemporaneous allies Coskinolinoides texanus Keijzer, Dictyoconus walnutensis (Carsey), Orbitolinopsis kiliani (Prever), Dictyoconus floridanus (Cole) subsp. elongata (Moullade) and Simplorbitolina manasi Ciry & Rat are described and some remarks are presented on the family Orbitolinidae.