Orthoceratoidea

The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longer juvenile portion and an inflated short adult portion that separate sometime in maturity.

thumb | right | alt=Rayonnoceras Espeyense Nautiloid. | Rayonnoceras Espeyense Nautiloid. Rayonnoceras is a genus of extinct cephalopods that lived around 325 million years ago during the Carboniferous. Although they resemble earlier actinocerids they are now thought to belong to the Pseudorthocerida

Orthoceratoidea, from Ancient Greek ὀρθός (orthós), meaning "straight", and κέρας (kéras), meaning "horn", is a major subclass of nautiloid cephalopods. Members of this subclass usually have orthoconic (straight) to slightly cyrtoconic (curved) shells, and central to subcentral siphuncles which may bear internal deposits. Orthoceratoids are also characterized by dorsomyarian muscle scars (a small number of large scars concentrated at the top of the body chamber), extensive cameral deposits, and calciosiphonate connecting rings with a porous and calcitic inner layer.