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]