Oonoceras (K288), from the Middle Ordovician to Middle Silurian of Europe and North America named by Hyatt in 1884, has a slender compressed exogastrically curved shell with a gradual expansion. The genotype, O. acinaces, is from Middle Silurian of central Europe. The sutures in Oonoceras form lateral lobes. The camerae (chambers) and body chamber are short. The siphuncle, which is close to the venter, is cyrtochoanitic, empty, with expanded segments. Oocerina (K288), named by Foerste, 1926, is a slender, exogastric genus, like Oonoceras, from the Upper Silurian of Europe, Russia and possibly North America. Oocerina differs primarily in having an actinosiphonate siphuncle composed of numuloidal (beaded) segments. Paroocerina (K288), named by Zhurevleva in 1961, is a slender oncoceratid from the Middle and Upper Silurian of eastern Europe and ex-USSR (Russia), similar to Oocerina, but with a hyponomic sinus. The genotypes of Oocerina and Paroocerina, respectively O.lentigratum and P. podolskensis, are from the Upper Silurian of central Europe and Russia
