  The layered cobaltite [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], often expressed as
the approximate formula Ca$_3$Co$_4$O$_9$, is a promising candidate for
efficient oxide thermoelectrics but an origin of its unusual thermoelectric
transport is still in debate. Here we investigate \textit{in-plane} anisotropy
of the transport properties in a broad temperature range to examine the
detailed conduction mechanism. The in-plane anisotropy between $a$ and $b$ axes
is clearly observed both in the resistivity and the thermopower, which is
qualitatively understood with a simple band structure of the triangular lattice
of Co ions derived from the angle-resolved photoemission spectroscopy
experiments. On the other hand, at high temperatures, the anisotropy becomes
smaller and the resistivity shows a temperature-independent behavior, both of
which indicate a hopping conduction of localized carriers. Thus the present
observations reveal a crossover from low-temperature itinerant to
high-temperature localized states, signifying both characters for the enhanced
thermopower.
