Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Hēlĭădĕs, um, f., = Ἡλιάδες, daughters of Helios and sisters of Phaëthon, who were changed into poplars (acc. to others, into alders) and their tears into amber, Hyg. Fab. 154; Ov. M. 2, 340 (cf. Verg. A. 10, 190, with id. E. 6, 62): nemus Heliadum, i. e. poplar- or alder-grove, Ov. M. 10, 91: Heliadum lacrimae, i. e. amber, id. ib. 10, 263; called also: Heliadum gemma, Mart. 9, 14, 6; hence: capaces Heliadum crustae, i. e. of amber, Juv. 5, 38.