Progressive meshes [Hoppe, H., August 1996] provide an attractive mechanism for transmitting 3D geometry over networks. Progressive meshes (PM) transmit a coarse initial mesh and refinements that can be applied to the initial mesh. However, these techniques assume a reliable network protocol such as TCP/IP is used for data transmission. When transmitting 3D geometry for graphical applications such as concurrent virtual environments distributed over wide area networks with some nodes potentially using wireless networks, many losses will occur. TCP/IP performance degrades in the presence of packet loss and multicast communication mechanisms typically do not provide reliable communication. These applications motivate the issue of transmitting geometric data over unreliable networks. In this paper, we discuss several errors that are caused when reconstructing PM geometries after some packets have been lost. We modify the PM data structures to improve robustness during packet loss. We use these modifications to improve a hybrid transmission technique that uses TCP to transmit the base mesh and portion of the initial mesh and then use UDP to transmit the remainder of the mesh to improve transmission performance.