It was thought at one time that Birds differed from Pterodactyls in the absence of teeth; but this only holds good for modern birds. If we go back to the Mesozoic age, we find that birds at that time did possess teeth. The oldest known bird, the Archæopteryx, had teeth in its jaws, and presents some very striking points of resemblance to reptiles. But if we compare the skeleton of a Pterodactyl (such as the P. spectabilis, now under consideration) with that of a bird, we shall see in its fore limbs certain very obvious differences. A bird never has more than three fingers in its hand or wing (viz. the thumb and next two digits), and the bones that support these fingers, corresponding to the bones in the palm of a human hand, are joined together. Neither of the bones corresponding to our fingers are much elongated, and of these the longest is that which corresponds to the thumb. But, on referring to the skeleton of our Pterodactyl, we find that it has four fingers, three of which are fairly developed and furnished with claws, while the outermost one is enormously elongated. This is believed to correspond to the little finger of the human hand, while the thumb seems to be represented by a small bone seen at the wrist. It was this long outside finger that chiefly served to support the flying membrane of the Pterodactyl. For this and other reasons, we are forbidden to look upon these creatures as relatives of birds. Again, all birds that can fly possess a "merrythought," or furculum; and such is not found in the Pterodactyl. As we have already remarked, some authorities, when these creatures were first brought to light, considered them to be mammals, as bats are. But equally conclusive arguments may be brought forward against that view. All mammals have the skull jointed to the backbone by two articulations, known as "condyles," whereas Pterodactyls have only one--in that respect resembling reptiles and birds. Also there are important differences in the structure of their jaws, showing that they are constructed on the reptilian plan, and not on that of the mammal. In order to give rapid movement to their wings during flight, they had powerful muscles in the region of the chest. These were attached to a shield-like breast-bone provided with a keel--as in birds. But this bird-like feature is only a necessary provision to enable them to fly, and does not point to any relationship. [Illustration: Fig. 35.--Skeleton of _Rhamphorhynchus phyllurus_, with delicate impressions of the flying membranes. (After Marsh.)]