@Anonymous
You are showing both curved lines and a mesh in your image. I will assume we can ignore the lines and that what you want is to bridge the gap between the surfaces of the opposing curved mesh or meshes. First, can you please clarify:
1. whether the upper and lower curved surfaces are part of the same mesh or are separate meshes?
2. whether the upper and lower meshes are joined at the ends, ie. do they make a single closed mesh?
3. are they currently an editable/Edit mesh or an editable/Edit poly or some other arrangement you have on your modifier stack?
4. Which way the surface normals are facing for each curved planar surface.
The above questions are relevant to the best workflow for you.
Briefly, to close the gap across these surfaces, there are 2 possible tools ("Cap" and "Bridge") but they both require that the surfaces are part of the same editable poly or editable mesh. But Cap will not work unless the surfaces are joined at the ends such that a complete closed edge border (in Max terms) is formed. And if you do not have a closed edge loop border, then "bridge" will work across opposing selected edges but will make a mess if the surface normals are both facing up.
So basically more information is needed from you unless this is enough info for you based on your knowledge of max to get one of the above tools to work.
