Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Sphinga (Spin-), ae, f., = Sphinx (late Lat.), Aug. Civ. Dei, 18, 13.
Plur., Hyg. Fab. 67; Sid. Ep. 5, 7; Isid. 12, 2, 32.