Lastly, we find in the Triassic rocks the remains of Reptiles belonging to the great Mesozoic order of the Deinosauria.  This order attains its maximum at a later period, and will be spoken of when the Jurassic and Cretaceous deposits come to be considered.  The chief interest of the Triassic Reptiles of this group arises from the fact that they are known by their footprints as well as by their bones; and a question has arisen whether the supposed footprints of birds which occur in the Trias have not really been produced by Deinosaurs.  This leads us, therefore, to speak at the same time as to the evidence which we have of the existence of the class of Birds during the Triassic period.  No actual bones of any bird have as yet been detected in any Triassic deposit; but we have tolerably clear evidence of their existence at this time in the form of footprints.  The impressions in question are found in considerable numbers in certain red sandstones of the age of the Trias in the valley of the Connecticut River, in the United States.  They vary much in size, and have evidently been produced by many different animals walking over long stretches of estuarine mud and sand exposed at low water.  The footprints now under consideration form a double series of single prints, and therefore, beyond all question, are the tracks of a biped—­that is, of an animal which walked upon two legs.  No living animals, save Man and the Birds, walk habitually on two legs; and there is, therefore, a prima facie presumption that the authors of these prints were Birds.  Moreover, each impression consists of the marks of three toes turned forwards (fig. 155), and therefore are precisely such as might be produced by Wading or Cursorial Birds.  Further, the impressions of the toes show exactly the same numerical progression in the number of the joints as is observable in living Birds—­that is to say, the innermost of the three toes consists of three joints, the middle one of four, and the outer one of five joints.  Taking this evidence collectively, it would have seemed, until lately, quite certain that these tracks could only have been formed by Birds.  It has, however, been shown that the Deinosaurian Reptiles possess, in some cases at any rate, some singularly bird-like characters, amongst which is the fact that the animal possessed the power of walking, temporarily at least, on its hind-legs, which were much longer and stronger than the fore-limbs, and which were sometimes furnished with no more than three toes.  As the bones and teeth of Deinosaurs have been found in the Triassic deposits of North America, it may be regarded as certain that some of the bipedal tracks originally ascribed to Birds must have really been produced by these Reptiles.  It seems at the same time almost a certainty that others of the three-toed impressions of the Connecticut sandstones were in truth produced by Birds, since it is doubtful if the bipedal mode