Moreover, this command aimed at the prevention of idolatry. For idolaters believed that if mice, lizards, or the like, which they used to sacrifice to the idols, fell into the vessels or into the water, these became more pleasing to the gods. Even now some women let down uncovered vessels in honor of the nocturnal deities which they call "Janae."
Erat etiam hoc praeceptum ad declinandam idololatriam, credebant enim idololatrae quod, si mures aut lacertae, vel aliquid huiusmodi, quae immolabant idolis, cito caderent in vasa vel in aquas, quod essent diis gratiosa. Adhuc etiam aliquae mulierculae vasa dimittunt discooperta in obsequium nocturnorum numinum, quae ianas vocant.
ST I-IIae.102.5 ad 4 (Latin)
That there would be pagan survivals into the thirteenth century is not at all surprising, but I have been struggling to link these 'Ianae' with anything. I don't think I have ever heard of them, and I can find nothing about them anywhere. Is this a folkloric corruption of some prior pagan deities (Lares and Penates, perhaps, or Janus), the collapse of some old practice into mere omen-collecting? Or is it more probably a textual corruption? What am I missing?