This is an impressive, 3,5" long specimen of Crotalocephalus Gibbus from Atchana, Morocco. Nice preparation and shell detail. It has been prepared in a "flying" pose by removing nearly all of the surrounding limestone. This leaves it floating above the rock on a pedestal. The preparation takes lot of time but the results are quite spectacular.
 
And They first appeared in the fossil record in the Cambrian period (521 million years ago) and became extinct during the Permian mass extinction (250 million years ago).