This is the cartoon version of The Magic School Bus In the Time of the Dinosaurs. (All the periods--Triassic, Jurassic, and Cretaceous--are visited in the book, but time constraints forced the episode to be limited to showing only the Cretaceous Period.) In this episode, the kids visit Ms. Frizzle's old friend, Dr. Carmina Skeledon (Rita Moreno), at a dinosaur dig. Dr. Skeledon lets Arnold see a fossilized dinosaur egg as the kids are leaving, and he still has it when they go back in time. An Ornithomimus steals the egg, and both Arnold and Phoebe chase after it. The rest of the class is left to find them, and in the end, Arnold squares off with a T. rex after being grown. (It resembles the one shown in Jurassic Park.)