How can I simplify selected polygons ?

[Cross22]

I created a semi-circle and then subdivided the top face a couple of times:

Now I would like to go the opposite way: weld together the selected coplanar polygons since I have no need for them. In this particular case I would like to just keep the outer ring and simplify the center to the minimum number of polys. The desired result would be something like this:

How can that be achieved in ZBrush? (In a traditional 3d modeller I would select all the middle vertices and weld them together).

Attachments

[Spyndel]

Tool> Geometry> Edge Loops> Delete Loops

Mask any rings you want to protect.

Use the transpose extrusion functions to extrude the precise number of rings you want after the fact, by ctrl-draging on the transpose line from the center of the disk in scale mode.

http://docs.pixologic.com/user-guide/3d-modeling/modeling-basics/transpose/