Shell Ornament 2 (revised)

A slightly more robust version of Shell Ornament 2, with thicker ribs and a bit less rotation. The rib width is less than 0.7mm at the centre, but the rib depth is ~1.3mm. I expect some fusing and loss of detail in the centre, but not too much, I hope. The previous version is still available at https://www.shapeways.com/model/66469/shell_ornament_2.html.