Preserved with rusty-colored limonite crystals, these are actually the molds of the shells of Flexicalymene trilobites.
Trilobites are a type of extinct arthropod most closely related to modern Horseshoe crabs.
Flexicalymene is a genus of trilobites of the order Phacopida, suborder Calymenina. They are found abundantly in North Africa and North America. Flexicalymene specimens can be mistaken for a few other Trilobite types. They are used as an index fossil in the Ordovician. Morocco and North America are particularly known for being rich with Flexicalymene fossils.