I purchased the specimen below from E-bay because the fossils looked very clean and with little restoration. There was no label included beyond "Onnia, Morocco" but a little searching on Paleodb.org gave me this: late Ordovician (Ashgill or Hirnantian stage), Ktaoua Grp, Upper Ktaoua Fm., Anti-Atlas Mountains, Morocco. This appears to be the same group that the Flexicalymene ouzregui I profiled earlier comes from but must be from a different horizon since the matrix is limestone rather than mudstone concretions.
These guys look very much like Cryptolithus sp. trilobites that are found here in North America but they are called Onnia superba when they come from Morocco.
There is an interesting paper available here: The Stratigraphical Distribution and Taxonomy of the Trilobite Onnia in the Type Onnian stage of the Uppermost Caradoc which has quite a bit of info and pictures of the species.