thalassoceratids are characterized by thick-discoidal to subglobular, involute shells with narrow or closed umbilici and serrate or digitate external lobes in the suture. this latter distinguishes them from the bisatoceratidae in which the external lobes are smooth. the ventral lobe is extremely wide; the height of median saddle may exceed half the height of the entire ventral lobe itself. some forms have ventrolateral grooves but spiral ornamentation is absent.