This paper is a revision of the standard chronostratigraphy of the Ord
ovician of the historical type area in England and Wales. The revision
is a response to the need for more precise definitions of series and
stages, especially for practical international correlation. In some ca
ses this has entailed moving horizons for Series bases away from those
in the classical sections. The scheme will be used as the standard fo
r a forthcoming revision of the Correlation Chart of the British Ordov
ician System. The Ordovician is divided into five series: Tremadoc, Ar
enig, Llanvirn, Caradoc and Ashgill. The Llanvirn is extended to inclu
de part of the classical Llandeilo Series, which is included within it
as a stage. Where stage subdivisions of standard series have not yet
been defined, as in the Tremadoc, they are proposed in this paper. How
ever, Bancroft's fine stadial divisions of the Caradoc, which have bee
n criticized for their brevity and local utility, are combined into fo
ur new stages, which will have wider application. The former Caradoc s
ubdivisions are retained as substages for the purposes of regional cor
relation.