The Collaborative International Dictionary of English v.0.48:
Septarium \Sep*ta"ri*um\, n.;pl. Septaria. [NL., fr. L. septum, saeptum, an inclosure, a partition, fr. sepire, saepire, to inclose.] (Geol.) A flattened concretionary nodule, usually of limestone, intersected within by cracks which are often filled with calcite, barite, or other minerals. [1913 Webster]