Orthoconic

An orthocone, loosely, may be thought of as a nautiloid shell, albeit somewhat larger and with a cone-shaped, straight formation as opposed to the nautiloid's coiled, curled shape. It was previously believed that these represented the most primitive form of nautiloid, however, it is now known that the earliest nautiloids had shells that were slightly curved. An orthoconic form evolved several times among cephalopods, and, among nautiloid cephalopods, is prevalent among the ellesmerocerids, endocerids, actinocerids, orthoceratoids, and bactritids.