Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
extentĭo, ōnis, f. [extendo], a stretching out, extension: capitis, Theod. Prisc. 2, 1, 3; Vitr. 7, praef. § 11.
Plur., Vitr. 9, 1, 13 al.