Curtoceras
Curtoceras is a genus in the tarphycerid family Trocholitidae found widespread in the late Early and Middle Ordovician of North America and northern Europe. "Curtoceras" has a shell that is gradually expanded, with half the fully mature body chamber divergent from the preceding volution. Whorl sections are near equidimentional with the inner margin (dosum) moderately impressed. The surface may be smooth or weakly ribbed. The siphuncle is ventral in the initial chamber and becomes dorsal after one volution. With the exception of the dorsal siphuncle, "Curtoceras" is somewhat similar to the tarphyceratid "Campbelloceras"