(Hays, 1970, emend. Foreman, 1975)

The top of the zone is defined by the morphotypic last appearance of Eucyrtidium matuyamai and is coincident with the lower limit of the Stylatractus universus Zone. The zone corresponds approximately to the Chi Zone in Antarctic sediments (Hays, 1965).

Events included in the zone are:

- Tm Lamprocyrtis heteroporos

- Bm Lamprocyrtis nigriniae

- Bm Lamprocyrtis neoheteroporos

- Tm Sphaeropyle robusta

The base of the zone is defined by the evolutionary transition from Eucyrtidium calvertense to Eucyrtidium matuyamai. The base of the zone approximates the base of the Pleistocene in the North Pacific.

Reference slides R 52.1, R 52.2 and R 52.3 from Sample 18-173-5-CC.