Space Trap by John Russel Fearn

When his fellow space-travelers revert to apes and his lovely fiancee vanishes, Ken Richmond grimly buckles on his ray gun and goes forth to break up an alarming conspiracy!

John Russell Fearn (1908–1960) was a British author and one of the first British writers to appear in American pulp science fiction magazines. Always a highly prolific author, he published not only under his own name, but also as Vargo Statten and other pseudonyms including Thornton Ayre, Polton Cross, Geoffrey Armstrong, John Cotton, Dennis Clive, Ephriam Winiki, Astron Del Martia (and others). He remains best known for his long-running Golden Amazon saga. At times these drew on the pulp traditions of Edgar Rice Burroughs. Fearn also wrote Westerns and crime fiction.