Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sellŭla, ae, f. dim. [sella], a little seat or stool; also, a sedan (post-Aug.; but cf. sellularius), Arn. 2, 23; Tac. H. 3, 84; Fronto Ep. ad M. Caes. 5, 44.