Thanks for that. I did see the diagram, where 'theta' is the angle from the polar axis. However I don't see the point of 'theta' in the stress equations 4.3.39 and 4.3.40. If there was a reference somewhere to longitudinal moments differing in orthogonal axial planes I could understand it, but there is not.
So what value should one assign to 'theta' for a conical shell ?