Wikipedia
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 Meneltarma in J. R. R. Tolkien's legendarium.
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 series. The stories were originally published in various fantasy magazines in the 1930s and 1940s, notably Weird Tales.
The book collects several prose poems, poems and tales, including stories from the author's Poseidonis cycle, set on a remnant of the lost continent of Atlantis.