Lewis & Short

ostentārĭus, a, um, adj. [ostentum],

  1. I. of or relating to prophetic indications, prophetic (post-class.): ostentaria novitas (al. ostentatoria), Mart. Cap. 2, § 151.
  2. II. Subst.: ostentārĭum, ii, n., a book of prognostics: Tuscum, Macr. S. 3, 7: arborarium, id. ib. 2, 16.