In other respects the two classes present many obvious differences, but these are more superficial than would be suspected at first glance. And Professor Huxley believes that, structurally, "reptiles and birds do really agree much more closely than birds with mammals, or reptiles with amphibians." While most existing birds differ thus widely from existing reptiles, the cursorial or struthious genera, comprising the Ostrich, Nandou, Emu, Cassowary, Apteryx, and the recently extinct Dinornis of New Zealand, come nearer to the reptiles in structure than any others. All of these birds are remarkable for the shortness of their wings, the absence of a crest or keel upon the breast-bone, and some peculiarities of the skull, which render them more peculiarly reptilian. But the gap between reptiles and birds is only slightly narrowed by their existence, and is somewhat unsatisfactory to those who advocate the development theory, which asserts that all animals have proceeded, by gradual modification, from a common stock. Traces had been discovered in the Mesozoic formations of certain Ornitholites, which were too imperfect to determine the affinities of the bird. But the calcareous mud of the ancient sea-bottom, which has hardened into the famous lithographic slate of Solenhofen, revealed to Hermann von Meyer, in 1861, first the impression of a feather, and, in the same year, the independent discovery of the skeleton of the bird itself, which Von Meyer had named _Archæopteryx lithographicus_. This relic of a far-distant age now adorns the British Museum. [Illustration: Fig. 1.--Archæopteryx lithographicus.] The skull of the Archæopteryx is almost lost, but the leg, the foot, the pelvis, the shoulder-girdle, and the feathers, as far as their structure can be made out, are completely those of existing birds. On the other hand, the tail is very long. Two digits of the manus have curved claws, and, to all appearance, the metacarpal bones are quite free and disunited, exhibiting, according to Professor Huxley, closer approximation to the reptilian structure than any existing bird. Mr. Evans has even detected that the mandibles were provided with a few slender teeth. On the other hand, the same writer points out certain peculiarities in the single reptile found also among the Solenhofen slates, which has been described and named _Compsognathus longipes_ by the late Andreas Wagner. This reptile he declares "to be a still nearer approximation to the missing link between reptiles and birds," thus narrowing the gap between the two classes.