Poseidonis, published in 1973, was the 59th volume of the Ballantine Adult Fantasy Series. Lin Carter edited the series as whole as well as individual volumes like this. Poseidonis was the fourth collection in the series by Clark Ashton Smith. It has excellent wraparound cover art by Gervasio Gallardo, an artist that Carter used for many Ballantine Adult Fantasy books.
Generally speaking, the four Ballantine Adult Fantasy books by Clark Ashton Smith each concerns the tales from a particular Smith location. This is certainly true of Zothique and Hyperborea, and to limited extent of Xiccarph. Like Xiccarph, however, only a few of the Poseidonis stories are actually set in Poseidonis, last island of the doomed and sinking Atlantis. Several other stories are set in Lemuria, and one in Ptolemides. Many stories after the first third of the book are what might be classed as "lost world" tales, in which a modern person (at least, of Smith's time) stumbles across a hidden realm of great antiquity and isolation.
Some of the stories in Poseidonis are good, and the best can stand with the best of the preceding three Clark Ashton Smith volumes in the series—I'm thinking in particular of the two tales of Malygris of Poseidon, "The Last Incantation" and "The Death of Malygris." These two are classic Smith, horrific and fantastic, with an echo of irony. One can almost imagine Smith tittering gleefully, like the narrator in an old-time horror movie. Perhaps half the book, though, concerns "lost worlds" of various types. I sense the influence of Lovecraft in some of these, particularly in the discovery of ancient, alien horrors still inhabiting the modern world. In my view, Smith is not so strong when he ventures into the"lost world" subgenre. Somehow, his writing is not scintillating in his usual manner.
Moreover, sometimes Smith strays into racism in the "lost world" stories. For example, in "The Root of Ampoi," the English protagonist remarks of some indigenous people, "... their features were good even according to European standards" (p. 164). The same story is heavily misogynistic. Can we blame Smith's time and place for these attitudes or is he picking up ideas from Lovecraft? I'm not a scholar of Smith or Lovecraft, and so the influence of Lovecraft in this respect counts as a wild guess.
Poseidonis as a whole is nevertheless very good. Interspersed between the stories are several poems and prose poems, which are excellent, and give us a good sense of Smith's range as a writer of the fantastic. Lin Carter again does a spectacular job of bringing together and organizing these stories. The book is worthwhile also for Carter's knowledgeable and thoughtful introduction and notes between sections of the book.