Ammonite

Ca 200 million years old

Madeleine finds an ammonite on a Normandy beach, now on her windowsill next to a crab claw and a few rocks.

Originating from within the bacritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian (circa 400 million years ago) and became extinct at the close of the Cretaceous along with the dinosaurs. The classification of ammonoids is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers. While nearly all nautiloids show gently curving sutures, the ammonoid suture line (the intersection of the septum with the outer shell) is variably folded, forming saddles (or peaks) and lobes (or valleys). Source: Wiki.