Another dinosaur, in some respects like the sauropod, was the stegosaur which grew to be twenty to thirty feet long, and heavier than the elephant. Unlike the sauropod, it had a short neck and was armored with a double row of great plates over its back, and sharp spines (one to three feet long) toward the end of the tail. The excessive stupidity of the creature is proved by the fact that its very simple brain weighed less than three ounces! Stegosaurs were plant eaters as indicated by the tooth structure, and, though they looked ferocious, they were probably not fighters, certainly at least nothing like the carnivorous types of dinosaurs we shall soon describe. [Illustration: Fig. 61.--Skeleton of the curious kind of dinosaur (stegosaur) of Mesozoic Age with great bony plates over the back. Length about thirty feet. (After Marsh.)] The ferocious dinosaurs of Mesozoic time were carnivorous, or flesh eaters, as shown by their numerous sharp teeth in relatively large heads. The largest known type is the tyrannosaur, an almost perfect skeleton of which, 40 feet long and 16 feet high, is mounted in the American Museum of Natural History in New York (Plate 17). So far as known, this was the largest carnivorous animal which ever walked on the earth. It is evident from the structure that it walked on its hind legs, the front ones having been much shorter and used something like arms. There were also various other smaller forms of two-legged flesh-eating dinosaurs, many of the wonderfully preserved tracks in the Triassic sandstones of the Connecticut River Valley having been made by such creatures when they walked around over soft, sandy mud flats at least eight or ten million years ago. The sandy mud with its tracks became somewhat hardened and then deeply buried under much more sediment which, through the ages, has been eroded off, thus exposing to view certain of the layers covered with tracks. Some bones of dinosaurs have also been found in the Connecticut Valley. [Illustration: Fig. 62.--Skeleton (restored) of a great two-legged dinosaur of the Mesozoic era. This type of plant eater grew to be fully twenty-five feet long. (After Marsh.)] Another remarkable type of two-legged dinosaur was much like the flesh eaters just described, but they were plant eaters. The largest of these grew to be 30 feet long and 15 to 20 feet high, comparable, therefore, to the tyrannosaur in size. A wonderful collection of almost perfect skeletons may be seen in the museum in Brussels, Belgium. In mining coal 1,000 feet below the surface in Belgium, twenty-two complete skeletons and several partial skeletons were found in an ancient river deposit of Cretaceous Age. A marvelously preserved specimen of one of these two-legged plant eaters found in Wyoming, has been called a "dinosaur mummy" because the skin and much of the flesh of the creature had shriveled down upon its bones. The minutest details of the texture of its skin are almost perfectly preserved.