  We have obtained intermediate-resolution optical spectroscopy of 44 candidate
very low mass members of the nearby young open cluster IC2391. Of these, 26
spectra are totally new, 14 were already analyzed in a previous paper and
another four are in common.
  These spectra, taken at the Cerro Tololo 4-meter and Magellan I and II
telescopes, allow us to confirm 33 of them as likely cluster members, based on
their spectral types, presence of Li, and Halpha emission. Among these new
cluster members is CTIO-160 (M7), the first IC2391 candidate to satisfy all
criteria for being a substellar member of the cluster, including detection of
the Li 6708A doublet. With the enlarged membership, we are able to locate the
lithium depletion boundary of the cluster more reliably than in the past. Based
on comparison to several theoretical models, we derive an age of 50+-5 Myr for
IC2391. We also estimate new ages for the Alpha Per and Pleiades clusters; our
ages are 85+-10 Myr and 130+-20 Myr, respectively. We derive an estimate of the
initial mass function of IC2391 that extends to below the substellar limit, and
compare it to those of other well-studied young open clusters. The index of the
power law mass function for IC2391 is alpha=0.96+-0.12, valid in the range 0.5
to 0.072 M(sun).
