Poseidonis is an imagined last remnant of the lost continent of Atlantis, mentioned by Algernon Blackwood in his short story, Sand (published in 1912), in his story collection, Four Weird Tales, and is also detailed in a series of short stories by Clark Ashton Smith. Smith based Poseidonis on Theosophical scriptures about Atlantis, (such as Secret Doctrine by Helena Blavatsky) and his concept of "the last isle of foundering Atlantis" is echoed by the Isle of Númenor in J. R. R. Tolkien's legendarium.
Stories in the Poseidonis Cycle
- "The Muse of Atlantis" (prose poem)
- "The Last Incantation"
- "The Death of Malygris"
- "Tolometh" (poem)
- "The Double Shadow"
- "A Voyage to Sfanomoë"
- "A Vintage from Atlantis"
- "Atlantis: a poem" (poem)
In the Pusadian series of short stories by L. Sprague de Camp, Poseidonis refers to the fictional lost island continent of Pusad, whose name was later corrupted to Poseidonis by the Greeks and whose fate was supposedly one basis for the Atlantis legend.
- William Scott Elliot, The Story of Atlantis, Theosophical Publishing Society, 1896, p.18