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Following is a list, undoubtedly incomplete, of early crossovers and cross-references in DC books.
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“The Super-Key to Fort Superman,” Action Comics # 241 (June 1958): Batman invades Superman’s Fortress of Solitude.
“Superboy Meets the Boy Wonder,” Adventure Comics # 253 (October 1958): Robin visits Smallville in the past.
“Superboy Meets the Young Green Arrow,” Adventure Comics # 258 (March 1959): Oliver Queen visits Smallville in the “present.”
“The Sleeping Beauty From Krypton,” Superman # 128 (April 1959): Bruce Wayne helps Superman protect his secret identity from Lois Lane.
“Supergirl Visits the 21st Century,” Action Comics # 255 (August 1959): Supergirl meets the young Tommy Tomorrow.
“The Second Life of Batman,” Batman # 127 (October 1959): Superman flashes into action in an alternate timeline in which Bruce Wayne’s parents were not murdered.
“Superman’s Other Life,” Superman # 132 (October 1959): Batman and Robin visit Superman at the Fortress of Solitude.
“The Mermaid From Metropolis,” Superman’s Girl Friend, Lois Lane # 12 (October 1959): Lois meets Aquaman.
“The Case of the Vanishing Arrows,” Adventure Comics # 266 (November 1959): The Green Arrow and Speedy receive a fan letter from Batman and meet Superman.
“The Manhunt on Land” and “The Underwater Archers,” Adventure Comics # 267 (December 1959): Aquaman moves ashore in pursuit of one super-criminal while The Green Arrow and Speedy go to sea in pursuit of another.
“The Hand From Nowhere,” Batman # 130 (March 1960): Luthor invades Batman’s turf.
“The Wolf-Man of Metropolis,” Superman’s Pal Jimmy Olsen # 44 (April 1960): Jimmy attends a masquerade party at which other attendees have costumed themselves as Superman, Batman, Robin, The Green Arrow, and Speedy.
“Superman’s Black Magic,” Superman # 138 (July 1960): Clark Kent attends a masquerade party at which other attendees (including Bruce Wayne) have costumed themselves as Batman, The Green Arrow, Speedy, and Wonder Woman.
“The Mermaid From Atlantis,” Superman # 138 (July 1960): Aquaman helps Superman protect his secret identity from Lois Lane.
“The Interplanetary Mission,” Adventure Comics # 275 (August 1960): Aquaman disposes of a kryptonite meteorite.
“The Underwater Olympics,” Adventure Comics # 277 (October 1960): Aquaman disposes of another kryptonite meteorite.
“Supergirl’s Busiest Day,” Action Comics # 270 (November 1960): Supergirl meets Batman and Robin.
“Jimmy’s Gorilla Identity,” Superman’s Pal Jimmy Olsen # 49 (December 1960): Jimmy meets Congo Bill and briefly becomes Congorilla.
“Flame-Dragon From Krypton,” Superman # 142 (January 1961): Bruce Wayne helps Superman protect his secret identity again.
“Superman’s Rival, Mental Men,” Action Comics # 272 (January 1961); Aquaman and Aqualad lend a hand.
“Brainiac’s Super-Revenge,” Action Comics # 280 (September 1961): Superman gets help from Congo Bill and Congorilla in derailing Brainiac’s latest plot.
“Superman Owes a Billion Dollars,” Superman # 148 (October 1961): Aquaman helps Superman pay his back taxes.