Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
mensurnus, a, um, adj. [mensis], monthly (for the more usual menstruus): spatium, * Cic. Inv. 1, 26, 39 (acc. to Prisc. p 595 P.; B. and K. menstrui spatii); so Novat. de Trinit.: divisiones. Cypr. Ep. 34.