The Morphologies and Lifetimes of Transitional Protoplanetary Disks
  I describe new constraints on the lifetimes and morphologies of transitional
protoplanetary disks from observations of 1--10 Myr old stars with the
\textit{Spitzer Space Telescope}. New Spitzer results clearly show evidence for
two kinds of transitional disks and thus two main disk evolutionary pathways:
disks which form an inner hole/gap and clear from the inside out and disks that
deplete more homologously. Analyzing the disk populations of 1--10 Myr old
clusters such as Taurus, IC 348, NGC 2362, and $\eta$ Cha show that the mean
transitional disk lifetime must be an appreciable fraction of the mean
protoplanetary disk lifetime: $\approx$ 1 Myr out of 3--5 Myr. The varieties of
transitional disk SEDs and correlations with other disk diagnostics are
consistent with multiple mechanisms responsible for clearing disks.
