Protcycloceratidae is an extinct family of slender, commonly annulate, members of the cephalopod order Ellesmerocerida that lived during the Early Ordovician. Protocycloceratidae, named by Kobayashi (1935), are characterized by transversely ribbed, elongate shells that may be straight or slightly curved, as to be either endogastric or exogastric. Although not universally accepted, Kröger and Mutvie (2005) revised the Protocycloceratidae to include only those ellesmerocerids that have straight transverse sutures and siphuncles with concave segments and thick connecting rings. Protocycloceratidae (sensu Flower, 1964) includes the follow genera, given with brief descriptions.