Thanks to recommendations from John Cowan and Brett in this thread, I’ve started Brian W. Aldiss’s Cryptozoic! (which turns out to have been waiting on my shelves to be read for almost fifty years, a new record); it occurred to me to wonder whether the title was a real word—I thought I’d seen it, but couldn’t remember what it meant—so I checked the OED, and it turns out there are two such words, Cryptozoic adj.1 and n (Geol.) “Designating the eon before the beginning of the Cambrian period (about 542 million years ago), equivalent to the Precambrian period; of or relating to this eon, which is characterized by a lack of obvious fossils (as contrasted with the Phanerozoic eon)” and cryptozoic adj.2 (Ecol.) “Designating animals that live in concealed habitats where they remain hidden from view, as in crevices or caves; of or relating to such animals.” That seemed annoying at first, but then I realized there would be few contexts in which the homonymy would cause confusion.
Incidentally, I learn from Wikipedia that the original U.K. title of the book was An Age. I guess that didn’t sound zippy enough for the U.S. market.