  Despite the extensive study of lithium depletion during pre-main-sequence
contraction, studies of individual stars show discrepancies between ages
determined from the HR diagram and ages determined from lithium depletion (Song
et al. 2002, White & Hillenbrand 2005) indicating open questions in the
pre-main-sequence evolutionary models. To further test these models, we present
high resolution spectra for members of the Beta Pictoris Moving Group (BPMG),
which is young and nearby. We measure equivalent widths of the 6707.8 Angstrom
Li I line in these stars and use them to determine lithium abundances. We
combine the lithium abundance with the predictions of pre-main-sequence
evolutionary models in order to calculate a lithium depletion age for each
star. We compare this age to the age predicted by the HR diagram of the same
model. We find that the evolutionary models under-predict the amount of lithium
depletion for the BPMG given its nominal HR diagram age of ~12 Myr (Zuckerman
et al. 2001), particularly for the mid-M stars, which have no observable Li I
line. This results in systematically older ages calculated from lithium
depletion isochrones than from the HR diagram. We suggest that this discrepancy
may be related to the discrepancy between measured M-dwarf radii and the
smaller radii predicted by evolutionary models.
