  The recently recognized class of "transitional disk" systems consists of
young starswith optically-thick outer disks but inner disks which are mostly
devoid of small dust. Here we introduce a further class of "pre-transitional
disks" with significant near-infrared excesses which indicate the presence of
an optically thick inner disk separated from an optically thick outer disk;
thus, the spectral energy distributions of pre-transitional disks suggest the
incipient development of disk gaps rather than inner holes. In UX Tau A, our
analysis of the Spitzer IRS spectrum finds that the near-infrared excess is
produced by an inner optically thick disk and a gap of ~56 AU is present. The
Spitzer IRS spectrum of LkCa 15 is suggestive of a gap of ~46 AU, confirming
previous millimeter imaging. In addition, UX Tau A contains crystalline
silicates in its disk at radii >~ 56 AU which poses a challenge to our
understanding of the production of this crystalline material. In contrast, LkCa
15's silicates are amorphous and pristine. UX Tau A and LkCa 15 increase our
knowledge of the diversity of dust clearing in low-mass star formation.
