
1) Fictional island
Found on
https://www.crosswordclues.com/clue/poseidonis

(Gr.), the last remaining portion of the great Atlantic Continent, the isle Atlantis referred to in the Critias of Plato.
Found on
http://blavatskyblogger.freeukisp.co.uk/quickblast%20W%20Q%20Judge%20Theoso

Poseidonis is an imagined last remnant of the lost continent of Atlantis, detailed in a series of short stories by Clark Ashton Smith. Smith based Poseidonis on Theosophical scriptures about Atlantis, (such as the The Secret Doctrine by Helena Blavatsky) and his concept of `the last isle of foundering Atlantis` is echoed by the Isle of Meneltarm.....
Found on
http://en.wikipedia.org/wiki/Poseidonis
[collection] Poseidonis is a collection of fantasy short stories by Clark Ashton Smith, edited by Lin Carter. It was first published in paperback by Ballantine Books as the fifty-ninth volume of its celebrated Ballantine Adult Fantasy series in July 1973. It was the fourth themed collection of Smith`s works assembled by Carter for the serie...
Found on
http://en.wikipedia.org/wiki/Poseidonis_(collection)
No exact match found.