series of fossils obtained from the Chalk of Western America by Professor Marsh. About 1871 a headless bird skeleton was discovered in the Upper Chalk of Western Kansas. 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

ᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤᏤ

FIG. II

missing parts of the Hesperornis duly obtained, but the remains of another and still more remarkable species (Ichthyornis

A

B dispar) of extinct birds were duly brought to light. By the new discovery both Hesperornis and Ichthyornis were found to possess teeth the former (Fig. 11) having its curved teeth (B) set in a common groove in the jaw-bones, whilst Ichthyornis (Fig. 12) makes a further advance towards perfection in dental apparatus, in that its twenty or so teeth of each jaw were lodged in distinct sockets. The importance of these facts as bearing on new and reptile-like characters in birds may be readily imagined. No living bird possesses the semblance of teeth, if we except the horny ridges of the Merganser's bill. Prior to Marsh's discoveries, no fossil bird was known to have been provided with true teeth -although indeed, in certain bird-remains, described by Owen, from the London clay (Eocene) of Sheppey, under the name of Odontopteryx (Fig. 13), the jaws were provided with bony projections. These projections, however, are not true teeth-which, as many readers may know, do not resemble bones, either in development or structure, being developed from the "gum" or lining membrane of the mouth, and not

FIG. 12.

from cartilage, as true bones usually are. Doubtless these projections aided Odontopteryx to catch its finny prey, as the horny ridges

Leaf-cutter, certainly possesses a double row of bony projections on its palate. But even this novel and unusual addition to the list of birdpossessions bears but a faint resemblance to the bony teeth of Odontopteryx, as these latter in turn are an entirely different and relatively modern feature of the bird type, when compared with the true teeth of their "American Cousins" of the Western Chalk.

The Ichthyornis, moreover, diminishes the distance betwixt birds and reptiles in yet another fashion-the joints of its spine (Fig. 12, B) were concave at either end (c), a conformation familiar to us in the joints of the fish-backbone, utterly unknown in living birds, but common enough in reptiles. This character alone, in the eyes of the naturalist, becomes invested with an importance hardly to be over-estimated as regards its reptilian relationships; and in Hesperornis also, certain features in addition to those already noted, show unmistakable marks of affinity to the reptile type. The teeth of this latter bird, set, as already remarked, in a common groove, strongly remind one of the manner in which the teeth of certain lizards are fixed in the jaws. Some of the teeth of this curious bird exhibit the manner in which one series of teeth was replaced by another-for, as most readers know, reptiles and fishes possess an unlimited supply and continual succession of teeth. The old teeth are ousted from their sockets by new teeth which are developed at their bases, and in the jaws of Hesperornis such a manner of tooth-formation, exactly imitating a common reptilian mode of renewal, is to be plainly seen. The tail of this great diver of the Chalk Seas was lastly, like that of the Archeopteryx of the Oolite epoch, very different from the caudal appendage of existing and of other fossil birds. At its middle and under parts the joints of the tail present long projections of flattened shape, which strongly suggest the idea of the tail having been a rigid unyielding member in so far as a side movement was concerned, but, like that of the beaver, being probably mobile in a vertical direction, and being thus of use in the diving movements of its possessor. The

last joints of the tail were massed together, but in a fashion different from that in which the "ploughshare-bone" of living birds is formed.

In so far as the birds themselves have rendered an account of their past history, it is clearly seen that their affinities to reptiles become very strongly marked in various directions, especially in the structure of the spine, and in the possession of true teeth. Ichthyornis, in the matter of its hollowed spine-bones (Fig. 12, B, c), and in that of its socketimplanted teeth, is a more modified and more truly reptile-like bird than Hesperornis. This latter again approaches much nearer reptiles than Odontopteryx (Fig. 13) of the London Clay, which latter, as becomes its nearer approach to the existing order of affairs, presents a less marked relationship with "the dragons of the prime.”

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But what evidence, we may lastly ask, do the reptiles afford on their side of any tendency towards the bird type? Have the reptiles remained as passive to advance and evolution, as they would appear at first sight to remain to-day; or does their history but repeat the changes and variations exhibited by their bird-neighbours? Let the history of the reptile-class in the past answer these queries. A considerable number of fossil reptiles are ranked to form a distinct order or division, marked by various near approaches to the structure of birds. A single example of this curious group will suffice to show the intermediate nature of its included forms. Once again the Lithographic Slates of Solenhofen yield a rich reward to geological investigation, and present us this time with the fossil skeleton of an animal, which in the flesh attained a length of about two feet. This is the Compsognathus (Fig. 14) of the geologist-a long-necked reptile, possessing a small head, the jaws of which, however, were armed with teeth. Its fore-limbs were short, its hind-limbs being long and bird-like. Like that of birds, its thigh bone (Fig. 15, B, fe) is shorter than its leg-bone. As in birds (Fig. 15 A), the upper half of the ankle-bone (Fig. 15 B, as, ca) unites with the lower part of the leg; but the lower half of the ankle (td) was not, as in birds, united with the instep-bones, or metatarsals, which are three (1, 2, 3, 4) in number, long and slender, to support the second, third, and fourth toes. A mere trace

FIG. 14.

of the instep-bone of the fifth toe exists, and the first or great toe is of small size. Looking at the structure of Compsognathus, little or no

doubt can be entertained that this reptile was capable of resting on its hind-limbs, in bird-like fashion, and of walking, or hopping, after the fashion of the feathered bipeds, to which indeed, by a use of the

imagination, strictly scientific, we may regard this reptile as having in due time given origin. It is unquestionably to the struthious birds, that is, to the ostriches and their allies, that this curious reptile bears the closest resemblance, and a comparative glance at the hinder extremities of the crocodile bird, and its reptilian neighbour, will suffice to show the marked resemblances and gradation which connect, and at the same time distinguish, this curious series of forms. The Compsognathus-limb stands intermediate betwixt the saurian (Fig. 15 c) and the bird (A); and, strictly judged, is comparable most nearly to that of the unborn chick. A glance at Fig. 15, in which the hind-limbs of the bird (A) Compsognathus and its allies (B) and the crocodile (c) are represented, will show the gradations, likenesses, and differences which exist between the three groups. Those "dragons of the prime," known as Iguanodon and Megalosaurus, from the Chalk and Oolite, are near relations of Compsognathus. And when we think of the size of these reptiles, which attained a length of from forty to sixty feet, and of the probability that, like their diminutive neighbour, they may have walked on two legs, the origin of the giant footprints (Figs. 8 and 9) of the Triassic Sandstones would appear to present no special difficulties in the way of satisfactory solution.

Mention must here be made of the curious Pterodactyls (Fig. 16), extinct reptiles of the Lias, Oolite, and Chalk, in which a wing-mem

brane or fold of skin, similar to that seen in bats, stretched from an outer and enormously elongated finger of each hand to the fore-limb, sides of the body and hind limbs, and between the hind-limbs and tail. By aid of this wing-membrane these literal "flying dragons" must have winged their way through the air with ease and speed. Their breast-bone was keeled like that of the bird (Fig. 6, f,g), their shoulder-girdle was bird-like, and their bones, as in birds, were hollow

was essentially bird-like, but the hind limbs and pelvis were reptilian, and unlike those of the bird; and these flying dragons possessed prominent jaws, usually furnished with socket-implanted teeth. The Pterodactyls are thus not markedly bird-like in any sense. They do not lie in the direct line or series of links between birds and reptiles, but apparently represent a bird-like but independent offshoot of the reptilian branch. In any view of their nature, however, they serve to show plainly and forcibly the modification of the reptilian type for flight; and it requires but a limited draft upon speculative philosophy to support the belief that reptile-modification in another direction, and certainly at an epoch anterior to the appearance of the Pterodactyls, probably produced the modified birds of which our existing ornithology is the collective product.

Space fails us in the endeavour to describe other examples of animals which from their anomalous structure seem to connect very diverse types of living forms. The mere mention of such fishes as Lepidosiren and Ceratodus (to be hereafter described) linking their class to that of the frogs or Amphibians; or of such a quadruped as the