Clarkoceras

Clarkoceras is a breviconic ellesmerocerid cephalopod, one of only two genera known to have crossed from the Late Cambrian, Trempealeauan, into the Early Ordovician, Gasconadian. (Flower 1964, Teichert 1988); the other being Ectenolites.

Description
Clarkoceras has a rapidly expanding, laterally compressed, relatively short, endogastrically cyrtoconic shell; The upper or dosal side is more strongly convex longitudinally than the lower or ventral side is concave. Sutures are essentially straight and close spaced indicating very short camerae (chambers). The siphuncle is relatively large, 0.3 the dorsoventral dimension and is ventral, although not necessarily marginal. (Flower 1964, Furnish and Glenister 1964).Septal necks are straight, reaching about half way back to the previous septum and the connecting rings are thick and layered.

Taxonomy
Clarkoceras was first thought by Clarke to be a pilocerid, who in 1897 give it the name Piloceras newton-winchelli. Ruedemann in 1905 recognized this as belonging to the Ellesmerocerida and renamed it Clarkoceras newton-winchelli (Clarke), which became the genotype.