Stegosaurus, from the Greek meaning "roof-lizard" because of the plates on its back, was a genus of large herbivorous dinosaurs from the Upper Jurassic of North America. It is among the most easily identifiable dinosaurs due to the distinctive double row of kite-shaped plates along the animal's back and the long spikes on its tail. Stegosaurus was the largest stegosaur, measuring up to forty feet in length and weighing up to 12,000 pounds. The plates of Stegosaurus were highly modified bony-cored scales, similar to those seen in crocodiles and many lizards today. The largest plates were found over the animal's hips and measured two feet by two feet. There has been some debate over the purpose of the plates, whith some proponents of the theory that the plates were meant to intimidate enemies. A later theory suggests that they were part of the dinosaur's cooling system. The tail appears to have acted as a weapon and included four to eight spikes each about two to three feet long, an arrangement known as a thagomizer. The skull of Stegosaurus was long and narrow, and its head was carried close to the ground, probably no higher than three feet off the ground. Interestingly, the brain of Stegosaurus was about the size of a walnut.