Some more specifications about what kind of crossovers I consider most worthy of inclusion here.
I wont be dealing with "allusion" crossovers. In my choice for #7, I listed Robert E. Howard's crossover between Bran Mak Morn and King Kull. But there's also a crossover of sorts between the mythos of Robert E. Howard and that of H.P. Lovecraft in the Bran Mak Morn story "Worms of the Earth." In this story, Bran needs the help of a race of subterranean serpent-men in order to unseat the invading Romans, and to do so he steals a prized idol from the serpent-men, so that they will attack the Romans for the return of their idol. In the course of events the story mentions one of Lovecraft's minor deific figures, "Dagon," as well as "the nightmare corpse-city of R'lyeh," which Lovecraft fans will recognize as the hangout of the Great Cthulhu.
But what I'm looking for are crossovers involving characters, or in a very few situations, characters and particular milieus, so no "allusion crossovers" will be cited-- although I must admit that the work of Lovecraft himself is highly dependent on such allusions. Some of Lovecraft's most ambitious stories weave ingenious webs of interrelationships between Lovecraft's parade of cosmic horrors-- the Old Ones, the Great Race, the sea-people of Innsmouth, and many more. The idea that such interconnections exist outside the sphere of humanity adds to the horror.
However, even in the best stories-- "Call of Cthulhu," "Shadow Out of Time," "Whisperer in Darkness"-- the reader is only told about the webs of influence connecting the various creatures and aliens. Most of these encounters are related long after the fact, so they do not project the unique flavor of the true crossover. Only one Lovecraft work includes a true crossover of characters conceived as independent presences, which I'll detail in my next post.
Also not considered here are "cameo crossovers," where characters have no interaction with a developed plot, but merely appear as "walk-ons," usually for the purpose of a quick joke. Here's an example of a "cameo crossover" from the 1961-63 comic strip SAM'S STRIP by Walker and Dumas, which specialized in having the strip's characters meet other comic-strip characters:
The practice of comic-strip characters meeting one another was rare in 1961, but has become largely accepted in modern newspaper comics. I should add, though, that a cameo is not the same as a parody, given that the parody reproduces aspects of a character as seen through a funhouse mirror-- and so clearly deviates from the history of the original character.
ADDENDA: I may as well add here another example of a crossover that seems less than impressive. I considered including Jules Verne's THE MYSTERIOUS ISLAND, in which Verne's castaway-characters encounter not only Captain Nemo-- as all viewers of the Ray Harryhausen film will know-- but also the character of Tom Ayrton from Verne's IN SEARCH OF THE CASTAWAYS. However, though Ayrton is an important character in that novel, he's not a central one; he exists as an impediment for the major characters, the group that is sincerely searching for the titular castaways. Ayrton's appearance is more than a cameo, but his significance is too minor to qualify the novel for the crossover-status I'm exploring.