cf. sigma 288 (with the alternative seir- spelling) and more generally sigma 478.
[1] Likewise in ps.-Herodian, Epimerismi 124.6, 179.12, and, according to Adler, the Ambrosian Lexicon 294. The form given here, nominative singular, is probably a generic lexical reference. The only quasi-literary attestation of it is Aetius 13.6.
[2] Likewise in Hesychius (sigma724), the Synagoge (sigma89), and Photius (sigma236 Theodoridis). This dative plural must be quoted from somewhere; extant possibilities, noted by Theodoridis, include Demosthenes 8.45 and 2 Ep. Peter 2.4 (variant reading there for sirai=s).