Description: A fourfold subdivision of the NP10 zonal interval is proposed based on calcareous nannofossil biostratigraphic events associated with lineages in the genus Tribrachiatus. This subdivision, based on a detailed analysis of DSDP Site 550, provides a reliable means of determining relative completeness of upper Paleocene-lower Eocene sections in deep-sea as well as epicontinental settings. By means of calibration to magnetochronology it affords a fine chronologic resolution for Biochron NP10. Correlation of planktonic foraminiferal Zones P5 to P6b to the proposed subzonal scheme as seen in Hole 550 is presented. Carbon isotopic trends in the four NP10 subzones as seen in Hole 550 are also given. The proposed subdivision rests largely upon the range of a new species described herein, Tribrachiatus digitalis. This species has a broad geographic distribution and a restricted range in Zone NP10. The concept of Zone NP10 was recently modified to conform to the proposal that Rhomboaster cuspis and Tribrachiatus bramlettei are synonymous, a view that is not shared here. The structural differences between nannoliths assigned to Rhomboaster and Tribrachiatus are clarified and it is shown that R. cuspis and T. bramlettei are discrete taxa. The main consequence of the new zonal scheme is that it allows the determination of a level equivalent to the base of the London Clay Formation in marine sections outside of northwestern Europe. It is inferred from this that the age of the basal transgression of the London Clay Formation (Walton Member) is 54.37 Ma.