  We present an analytic calculation of the thermonuclear depletion of lithium
in contracting, fully convective, pre-main sequence stars of mass M < 0.5
M_sun. Previous numerical work relies on still-uncertain physics (atmospheric
opacities and convection, in particular) to calculate the effective temperature
as a unique function of stellar mass. We assume that the star's effective
temperature, T_eff, is fixed during Hayashi contraction and allow its actual
value to be a free parameter constrained by observation. Using this
approximation, we compute lithium burning analytically and explore the
dependence of lithium depletion on T_eff, M, and composition. Our calculations
yield the radius, age, and luminosity of a pre-main sequence star as a function
of lithium depletion. This allows for more direct comparisons to observations
of lithium depleted stars. Our results agree with those numerical calculations
that explicitly determine stellar structure during Hayashi contraction. In
agreement with Basri, Marcy, and Graham (1996), we show that the absence of
lithium in the Pleiades star HHJ 3 implies that it is older than 100 Myr. We
also suggest a generalized method for dating galactic clusters younger than 100
Myr (i.e., those with contracting stars of M > 0.08 M_sun) and for constraining
the masses of lithium depleted stars.
