The Eocene epoch is usually broken into Early and Late, or - more usually - Early, Middle, and Late subdivisions. The corresponding rocks are referred to as Lower, Middle, and Upper Eocene. The Faunal stages from youngest to oldest are:

Priabonian (37.2±0.1 – 33.9±0.1 Ma)
Bartonian (40.4±0.2 – 37.2±0.1 Ma)
Lutetian (48.6±0.2 – 40.4±0.2 Ma)
Ypresian (55.8±0.2 – 48.6±0.2 Ma)

The Ypresian and occasionally the Lutetian constitute the Early, the Priabonian and sometimes the Bartonian the Late state; alternatively, the Lutetian and Bartonian are united as the Middle Eocene.