Eleusian vs. Eleusinian (and, to a lesser extent, Elysian)

Both Eleusian and Eleusinian are used in relation to mysteries. I've only seen Elysian used in relation to Elysian Fields.

Given that the suffix -ian denotes "of or belonging to," I'm wondering if there is a rationale behind the use of Eleusinian. If I wanted to describe something 'of or belonging to' Eleusis, it would seem that Eleusian would be the most obvious choice.

Is Eleusinian used specifically to denote a meaning broader than originating from Eleusis? I'm wondering what (if any) the linguistic basis is for this.

Solution 1:

To be more precise, the English word Eleusinian can be traced to the ancient Greek Ἐλευσίς (Elefsis, eu is read as fin Greek) which becomes Ἐλευσῖνος (Elefsinos) in the genitive and provides the -in- in Eleusinian. Ἐλευσίς is a place name (Ελευσίνα, pronounced Elefsina, in modern Greek) and since the mysteries are the mysteries of Eleusis, in Greek the genitive is used so they are the mysteries of Ἐλευσῖνος => Eleusinian.

Eleusian is simply wrong and Elysian is completely unrelated, it comes from the ancient Greek Ἠλύσιον which is the name of a place in the underworld (Hades). It was, essentially, the nice neighborhood of Hell, the equivalent of Heaven. It has absolutely nothing to do with Eleysis or its mysteries.

Solution 2:

According to American Heritage and Random House, Eleusinian is from Latin Eleusīni(us) “of Eleusis,” which is in turn from Greek Eleusinios. The stem change doesn't indicate any special meaning; that's simply how it appears in the source languages. (As terdon notes, it's from the genitive inflection of the name.)

Eleusian does not appear in dictionaries, and very rarely appears in English writing. It's most likely a misspelling.