It came as a pleasant surprise to find a lot of remnants of fossils in the large capping stones that make up the walls around the Siene River that flows near Notre Dame Cathedral and St. Michel Fountain in the city. The rock usually has a lot of empty cavities of where the fossil shell once existed. Spiral gastropods seems to have been the most common fossil in the large rock slabs used.
Here are some pictures of gastropod fossils from the National Museum of Natural History in the Gallery of Comparative Anatomy & Paleontology. This spirally snails are called Potamides lapidum. Maybe they are the species in the rock used to create the containment walls for the Siene River going through Paris?
The next two pictures of a gastropod fossil slab with the fossil cavities and a casting showing the shell shapes. There was a text in French describing it saying this:
Plaque de Calcaire siliceux avec moules externes de Potamides submargaritaceus BRAUN. Le moulage en plâtre montre la morphologie réele des individus. Oligocène, Aix (B. -du -Rh.)
A Google translation reveals this which might have taken some words out of context:
Siliceous limestone plate with mussels external Potamida submargaritaceus BRAUN. The plaster cast shows the morphology of real people. Oligocene, Aix (Bouches-du-Rh.)