Atlan.

Paperback Library, 1968. Mass market paperback. Depending on how you count, this is either the 2nd 3rd book in a series (the first was republished as two separate novels after the initial appearance). So we'll call this Book 3 in the 5-book "Atlan / Cija" cycle. Set in prehistoric South America and in the mythical world of Atlantis. The books in the series are: "The Serpent (1963); "The Dragon" (1963); "Atlan" (1965); "The City" (1966); and "Some Summer Lands" (1977).