_Appearance and Habits of Allosaurus._ A study of the mechanism of the Allosaurus skeleton shows us in the first place that the animal is balanced on the hind limbs, the long heavy tail making an adequate counterpoise for the short compact body and head. The hind limbs are nine feet in length when extended, about equal to the length of the body and neck, and the bones are massively proportioned. When the thigh bone is set in its normal position, as indicated by the position of the scars and processes for attachment of the principal muscles (see under Brontosaurus for the method used to determine this), the knee bends forward as in mammals and birds, not outward as in most modern reptiles. The articulations of the foot bones show that the animal rested upon the ends of the metapodials, as birds and many mammals do, not upon the sole of the foot like crocodiles or lizards. The flat vertebral joints show that the short compact body was not as flexible as the longer body of crocodiles or lizards, in which the articulations are of the ball and socket type showing that in them this region was very flexible. The tail also shows a limited flexibility. It could not be curled or thrown over the back, but projected out behind the animal, swinging from side to side or up and down as much as was needed for balance. The curvature of the ribs shows that the body was narrow and deep, unlike the broad flattened body of the crocodile or the less flattened but still broad body of the lizard. The loose hung jaw, articulated far back, shows by the set of its muscles that it was capable of an enormous gape; while in the skull there is evidence of a limited movement of the upper jaw on the cranial portion, intended probably to assist in the swallowing of large objects, like the double jointed jaw of a snake. As to the nature of the skin we have no exact knowledge. We may be sure that it had no bony armor like the crocodile, for remains of any such armor could not fail to be preserved with the skeletons, as it always is in fossil crocodiles or turtles. Perhaps it was scaly like the skin of lizards and snakes, for the horny scales of the body are not preserved in fossil skeletons of these reptiles. But if so we might expect from the analogy of the lizard that the scales of the head would be ossified and preserved in the fossil; and there is nothing of this kind in the Carnivorous Dinosaurs. We can exclude feathers from consideration, for these dinosaurs have no affinities to birds, and there is no evidence for feathers in any dinosaur. Probably the best evidence is that of the Trachodon or duck-billed dinosaur although this animal was but distantly related to the Allosaurus. In Trachodon (see p. 94), we know that the skin bore neither feathers nor overlapping scales but had a curiously patterned mosaic of tiny polygonal plates and was thin and quite flexible. Some such type of skin as this, in default of better evidence, we may ascribe to the Allosaurus.