Dinosaurs are different, from other reptiles and from each other. And this lesson in taxonomy is also of a different order from other children's dinosaur books, including Aliki's own. The artist's familiar museum setting, with kid visitors dashing about amid the giant skeletons, gives this as frisky and bright a look as ever. But the information conveyed via the kids' jaunty speech-balloons can be as dry and weighty as some of the bones. ""All archosaurs have diapsid skulls,"" expounds an especially brainy tot, who is soon seen pointing out, ""That's Iguanodon's predentary sticking out in front of its teeth."" Other young observers oblige with more familiar definitions (""a meat eater is a carnivore"" and ""if you're two-legged, you're a biped""). But casual readers may well be taken aback by an early page of jawbreakers identifying Tyrannosaurus and Iguanodon as saurischians and ornithischians, respectively, and placing them within the archosauria group along with thecodonts, crocodillians, and pterosaurs. Whew! Aliki goes on to explain the different hip and jaw structures of the two orders, then divides the orders into suborders and breaks down some suborders even further. Thus when Corythosaurus, Stegosaurus, and a few others are at last paraded by, they come classified, with orienting flags at the page corners denoting ""the order, the suborder, the hip symbol, and green means plant-eater,"" as our little professor explains when this device first appears. There is also a summarizing outline on the final page. Readers serious enough for this systematic approach are given every help in getting it straight. It's all a little overwhelming for a first encounter, but for a serious reader already hooked by the subject, or for a class or family planning a museum visit, the rewards can be substantial.