Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
arcĕra, ae, f. [arca, Curt.], a covered carriage for sick persons: quod ex tabulis vehiculum erat factum ut arca, arcera dictum, Varr. L. L. 5, § 140 Müll.; Gell. 20, 1, 29; Non. p. 55, 26. So in the laws of the XII. Tables, Fragm. ap. Gell. 20, 1, 25; Varr. ap. Non. l. l. Acc. to Nonius ib. this word was found also in Cicero. At a later period the litter (lectica, sella) came into use, and hence arcera disappeared from the language.