[ODE] Basic problem - getting "use of undefined type 'dxGeom'"

[Megan Fox]

This is a ridiculously simple problem, and I know I must be making a
mistake somewhere, but... regardless, I'm getting this error: "use of
undefined type 'dxGeom'"

On this line of code:

geom = nOpende::CreateBox(...);
geom->geomID = geom; (<-- this one)

Where geom is of the following struct:

struct GeomContainer
    {
        dGeomID geomID;
        GeomContainer *next;
    };


All I'm trying to do is store the dGeomID (pointer to dxGeom) so that
I can destroy it later.  Nothing fancy at all.  I've tried casting it
to a void* (and changing GeomContainer to a void* internally), I've
tried pre-defining dxGeom, etc.  No dice.

What am I doing wrong, here?

Thanks,
-Megan Fox