It's not out in the open like Notre-Dame
Shell from a giant clam; there's two, one for each shell-half. The base was sculpted by Pigalle.
Would it be wrong to say that it's a huge organ?
A few blocks from Notre-Dame, Saint'Severin is a small (!) cathedral that's about seven hundred years old, at least this version.
The lighting was poor for indoor pics, and I didn't have a tripod to take pics without a flash, so they're mostly fuzzy.