(Riedel and Sanfilippo, 1970)
The top of the zone is defined by the earliest evolutionary appearance of Didymocyrtis penultima and is coincident with the lower limit of the Didymocyrtis penultima Zone.
The base of the zone is defined by the first evolutionary appearance of Didymocyrtis antepenultima and is coincident with the upper limit of the Diartus petterssoni Zone.
Although equivalent to the low-latitude zone of the same name, the aspect of the fauna in middle latitudes is often different and important marker species are frequently missing.
Reference slides R 57.1, R 57.2 and R 57.3 from Sample 18-173-18-35, 35-40 cm.