producing the so-called "tarso-metatarsal" bone (Fig. 76, w). It is this bone which becomes so greatly elongated in the waders, such as the storks and ibises. As seen in the young fowl (Fig. 77B), the shin or leg-bone (1) bears at its lower extremity the "astragalus" (a) of the

ankle, shortly to be firmly united to the leg by bony union (A). The latter condition is seen in the lefthand figure (A), where the astragalus (a) has become united to the tibia, or chief leg-bone (t); the other bone of the leg, or fibula (f), being rudimentary. Such a complete union of ankle-bones with the leg is not seen in any living reptiles (see Fig. 85, c). Whilst the latter have four toes as their least complement, birds have never more than four, the fifth toe being invariably wanting. And whilst in birds, the bones of the instep unite with the lower half of the ankle to form a single bone (Figs. 85, A, tm, and 76, w), in reptiles the instep bones (or metatarsals)

(Fig. 85 C, 1, 2, 3, 4) are not united together, and are distinct from those of the ankle.

Thus much for dry details. The reader who has taken the trouble to follow this category of the personal characters of birds as compared with those of reptiles, will probably find that the somewhat extended examination will assist his comprehension of certain abnormalities in the structure of several extinct forms of bird and reptilian life; since many of the characteristic features of each class just detailed will be found to have been curiously modified and often united in the "missing links" which bind these two groups of animals together. It may be firstly asserted that the ostriches, cassowaries, and their relatives, differ from all other birds in possessing a flat shield-like breast-bone instead of the normal "keeled" structure (Fig. 76, f, g) proper to the class. Their "merrythought" is likewise incomplete,

and their haunch-bones are united below or in front, instead of remaining open as in other birds. But he would be worse than an over-bold zoologist who would venture to maintain that such points of difference meant more than the merest tendency reptilewards; and the ostriches and their neighbours can hardly be denominated links which appreciably narrow the gulf betwixt reptiles and their bird kith and kin. But presuming that the zoologist, dealing with the birds of to-day, refuses f assent to the idea that he can supply us with missing links between reptiles and birds, can the contents of the geologist's aviary be shown to be better adapted to supply the gap? Research here may proceed in two directions. Either we may try to discover if any extinct birds are nearer reptiles than their living allies; or, we may endeavour to ascertain if any fossil reptiles exhibit a closer relationship with birds than the reptiles of to-day. We may very profitably discuss, in brief detail, both aspects of the case.

B

FIG. 77.-LEG AND

ANKLE OF BIRD. A, in the full-grown bird; B, in the young

state.

Fossil birds make their first appearance in the Upper Oolite

rocks-formations lying in their natural order just below the chalk.

Prior to the Oolitic epoch, however, and in the Triassic rocks of America, certain large footprints (Figs. 78 and 79), supposed by some authorities to be those of birds, are found. But these footprints may, at the same time, be those of reptiles, and it is safer at present to hold their exact nature as undetermined, and to assert that the first unmistakable bird-fossil belongs to the Oolitic period.

The Lithographic Slates of Solenhofen, in Bavaria, are rocks resulting from the consolidation of the finely powdered mud which once coated an ancient Oolitic sea- or lake-bed. On this fine-grained deposit, belonging to the Upper Oolite series, the merest traces and most delicate impressions of living organisms have been preserved -the impress of even a filmy jelly-fish having thus been brought to light. In 1861 the impression of a single feather was found, and later

FIG. 80.-FOSSIL REMAINS OF ARCHAEOPTERYX,

on in the same year, a Dr. Häberlein of Pappenheim brought to light the fragments of a skeleton which was soon discovered to be of a thoroughly unique kind. This scientific treasure was duly purchased for the British Museum, and was named the Archaeopteryx macrura (Fig. 80). The skull of Archaeopteryx was wanting in this first specimen, but the leg, foot, pelvis, tail, shoulder, and some of the feathers are well preserved, and by these relics the materials for a strange history was in part supplied. Of the bird-nature of this creature no doubt exists. In the matter of its feathers and feet it is wholly bird-like. But it is also discovered to differ very materially from all known birds. Thus, firstly, Archæopteryx possessed a long tail (Fig. 80), exactly resembling that of a lizard, consisting of some twenty

joints, each of which supported a pair of quill feathers. Then, secondly, no ploughshare-bone (Fig. 76 d) was developed. The fingers, united by bony union in existing birds, were free and reptilelike in Archeopteryx, and, whatever their number may have been, it is certain that these fingers were provided with reptile-like claws, such as are seen in no living bird.

Such were the details of Archeopteryx structure at hand till within the last three years or so. In 1879 Professor Carl Vogt made a communication concerning a fresh specimen of this ancient bird, found in the same deposits which afforded the previous specimen. The new specimen was singularly complete; and its wings were unfolded, as if death and fossilisation had overtaken it in the act of flight. Its examination revealed certain startling features, which only serve to confirm in an unmistakable manner the thoroughly "intermediate" nature of this animal. Its upper jaw bore two small conical teeth; the breast-bone is "reduced to zero ;" and whilst its armbones" present no features peculiar to reptiles or to birds," its hand can be compared neither to that of a bird nor to that of a pterodactyl, but to that of a three-toed lizard. "If the feathers had not been preserved," says Vogt, "no one could have ever suspected that, from the examination of the skeleton alone of Archeopteryx, this animal was furnished with wings when alive." Head, neck, chest and ribs, tail, shoulder-girdle, and arm or wing, are all built on a reptilian type; the haunch is more reptilian than bird-like; but the hind limbs are those of a bird. The reptile characters unquestionably predominate in the skeleton, just as the bird-characters come to the front in the feathers.

Professor Vogt strenuously asserts that a study of Archaeopteryx shows that it is neither bird nor reptile, but that it is a decided "link" betwixt the two classes. It is a bird by its feathers and hind limbs; it is a 'reptile by the rest of its structure; and it is, moreover, a bird only in so far as we regard its type as having emerged from a reptilian stock. The birds to be presently described from the American Chalk are later developments. As such, they are nearer the birds of to-day; but they retain the reptilian teeth, whilst the rest of their organisation has been evolved along the lines of bird-structure. Professor Vogt further insists on the fact that the adaptation to flight is not necessarily combined with an erect position, since the extinct pterodactyls and the living bats illustrate cases in which that position was and is not maintained. The bird-like hind feet of the Archeopteryx must be viewed as having been independent of flight, and as related to the possibility of sustaining the body on the hinder feet alone. In other words, we are not specially entitled to concern ourselves with the question of flight in this ancient animal; and the consideration is worth attention

in dealing with the affinities of the Archaeopteryx. Finally, as Vogt points out, there is a complete affinity betwixt the scale of the reptile and the feather of the bird. The feather is, in fact, the further modification of the scale; and we may, therefore, "imagine the ancestors of the Archæopteryx as lizard-like terrestrial reptiles, having feet with fine, hooked, free digits, showing no modification in

FIG. 81.

their skeleton, but having the skin furnished at different points with elongated warts, downy plumes, and rudimentary feathers, not yet fitted for flight, but susceptible of further development in the course of generations."

But that this odd relic of the Oolite leads us decidedly in the direction of the reptiles by its tail and its hand there can exist no reasonable doubt. Scepticism may exist on this latter point, but the doubt is neither of a learned nor of a scientific kind. We may not say that Archæopteryx actually leads us from any one bird to any one group of B reptiles. It rather stands intermediately and alone; but even in its solitary position it certainly makes the gulf betwixt the two classes seem less formidable.

Next in order from the aviary of the geologist may be produced evidence of the existence of reptilelike birds in a most interesting series of fossils obtained from the Chalk of Western America by Professor A Marsh. About 1871 a headless bird skeleton was discovered in the HESPERORNIS Upper Chalk of Western Kansas. JAW. This bird evidently resembled our living divers, and was duly christened Hesperornis regalis. Like our living ostriches, emeus, and their allies, this extinct bird possessed no keel on its breast-bone. It had the merest rudiments of wings; and certain reptile-like resemblances seen in its haunch-bones made geologists naturally anxious for the realisation of their hopes in the discovery of a complete skeleton. In 1872 fresh discoveries rewarded the patient and indefatigable search of Professor Marsh. Not only were the missing parts of the Hesperornis duly obtained, but the remains of another and still more remarkable species (Ichthyornis dispar) of extinct birds, were duly brought to

FIG. 82. ICHTHYORNIS JAW.