The primary headword has epsilon iota in its penultimate syllable, as in (e.g.) Homer, Odyssey 8.362, Iliad 3.424. The alternative spelling with eta, in the second part of the entry, occurs at Hesiod, Theogony 200, and has suggested an etymological connection with the severed genitals of Kronos (o(/ti mhde/wn e)cefaa/nqh). Note that in any event versions with -mm- (from *-sm-) are attested.
[1] Likewise or similarly in other lexica; references at Photius phi186 Theodoridis.