That is'nt transparent. That is because the top and the bottom of bonnet has the same material/skin.
For Default Scanline Renderer of 3ds max, you can use the 'double sided' material to solve this problem, which you can assign different material/skin to the top and bottom side of a single sided object.
For other renderer, 'double sided' material may not be supported. however,
you could easily add a shell modifier to the detached bonnet so it turns into 'solid' object, which you can then select the top side of the bonnet object to have a different material/skin to the bottom side of the bonnet.