Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. cassis, ĭdis (access. form cassĭda, ae, like chlamyda from chlamys, Prop. 3 (4), 11, 15; Verg. A. 11, 775; cf. Prob. II. p. 1473 P. Lind.; Charis. p. 80 P.; Prisc. p. 698 ib. Also Paul. ex Fest. p. 48, instead of cassilam, the reading should be cassidam), f. [Etruscan, acc. to Isid. Orig. 18, 14, 1; cf. Müll. Etrusk. 1, p. 392; cf. Sanscr. khad, to cover, and Lat. castrum, casa], a helmet, commonly of metal (as galea was of leather, Isid. Orig. 18, 14, 1; cf. Tac. G. 6), Plaut. Trin. 3, 2, 100; Caes. B. G. 7, 45; Ov. M. 12, 89; 13, 107: fracta, Juv. 10, 134: caelata, id. 11, 103.
But also promiscuously for galea, Ov. M. 8, 25 (cf.: aerea galea, Verg. A. 5, 490).