Polished fossil stone - very good condition - large dimensions Orthoceras Middle Ordovician Supplied with metal base Maximum dimensions 29 x 25 cm This nautiloid cephalopod had a conical, cylindrical, cone-shaped shell with closely spaced chambers, containing a hollow, spherical septum, with a connecting tube in the centre, the sipho. The length of the shell was approx. 15 cm. Since the orthocere is a mollusque, the only petrified part is the "vertibra" of the squid.