DRAGONS OF THE AIR

FIG. 47. RHAMPHORHYNCHUS PHYLLURUS
SHOWING THE PRESERVATION OF THE WING MEMBRANES

From the Lithographic slate of Eichstädt, Bavaria

Frontispiece

DRAGONS OF THE AIR

AN ACCOUNT OF
EXTINCT FLYING REPTILES

BY

H. G. SEELEY, F.R.S.

PROFESSOR OF GEOLOGY IN KING'S COLLEGE, LONDON; LECTURER ON GEOLOGY
AND MINERALOGY IN THE ROYAL INDIAN ENGINEERING COLLEGE

WITH EIGHTY ILLUSTRATIONS

NEW YORK: D. APPLETON & CO.
LONDON: METHUEN & CO.
1901

PREFACE

I was a student of law at a time when Sir Richard Owen was lecturing on Extinct Fossil Reptiles. The skill of the great master, who built bones together as a child builds with a box of bricks, taught me that the laws which determine the forms of animals were less understood at that time than the laws which govern the relations of men in their country. The laws of Nature promised a better return of new knowledge for reasonable study. A lecture on Flying Reptiles determined me to attempt to fathom the mysteries which gave new types of life to the Earth and afterwards took them away.

Thus I became the very humble servant of the Dragons of the Air. Knowing but little about them I went to Cambridge, and for ten years worked with the Professor of Geology, the late Rev. Adam Sedgwick, LL.D., F.R.S., in gathering their bones from the so-called Cambridge Coprolite bed, the Cambridge Greensand. The bones came in thousands, battered and broken, but instructive as better materials might not have been. My rooms became filled with remains of existing birds, lizards, and mammals, which threw light on the astonishing collection of old bones which I assisted in bringing together for the University.

In time I had something to say about Flying Animals which was new. The story was told in the theatre of the Royal Institution, in a series of lectures. Some of them were repeated in several English towns. There was still much to learn of foreign forms of flying animals; but at last, with the aid of the Government grant administered by the Royal Society, and the chiefs of the great Continental museums, I saw all the specimens in Europe.

So I have again written out my lectures, with the aid of the latest discoveries, and the story of animal structure has lost nothing in interest as a twice-told tale. It still presents in epitome the story of life on the Earth. He who understands whence the Flying Reptiles came, how they endured, and disappeared from the Earth, has solved some of the greatest mysteries of life. I have only contributed something towards solving the problems.

In telling my story, chiefly of facts in Nature, an attempt is made to show how a naturalist does his work, in the hope that perhaps a few readers will find happiness in following the workings of the laws of life. Such an illumination has proved to many worth seeking, a solid return for labour, which is not to be marketed on the Exchange, but may be taken freely without exhausting the treasury of Nature's truths. Such outlines of knowledge as here are offered to a larger public, may also, I believe, be acceptable to students of science and scientific men.

The drawings given in illustration of the text have been made for me by Miss E. B. Seeley.

H. G. S.

Kensington, May, 1901

CONTENTS

LIST OF ILLUSTRATIONS

These figures are greatly reduced in size, and when two or more bones are shown in the same figure all are brought to the same size to facilitate the comparison.

DRAGONS OF THE AIR

CHAPTER I
FLYING REPTILES

The history of life on the earth during the epochs of geological time unfolds no more wonderful discovery among types of animals which have become extinct than the family of fossils known as flying reptiles. Its coming into existence, its structure, and passing away from the living world are among the great mysteries of Nature.

The animals are astonishing in their plan of construction. In aspect they are unlike birds and beasts which, in this age, hover over land and sea. They gather into themselves in the body of a single individual, structures which, at the present day, are among the most distinctive characters of certain mammals, birds, and reptiles.

The name "flying reptile" expresses this anomaly. Its invention is due to the genius of the great French naturalist Cuvier, who was the first to realise that this extinct animal, entombed in slabs of stone, is one of the wonders of the world.

The word "reptile" has impressed the imagination with unpleasant sound, even when the habits of the animals it indicates are unknown. It is familiarly associated with life which is reputed venomous, and is creeping and cold. Its common type, the serpent, in many parts of the world takes a yearly toll of victims from man and beast, and has become the representative of silent, active strength, dreaded craft, and danger.

Science uses the word "reptile" in a more exact way, to define the assemblage of cold-blooded animals which in familiar description are separately named serpents, lizards, turtles, hatteria, and crocodiles.

Turtles and the rest of them survive from great geological antiquity. They present from age to age diversity of aspect and habit, and in unexpected differences of outward proportion of the body show how the laws of life have preserved each animal type. For the vital organs which constitute each animal a reptile, and the distinctive bony structures with which they are associated, remain unaffected, or but little modified, by the animal's external change in appearance.

The creeping reptile is commonly imagined as the antithesis of the bird. For the bird overcomes the forces that hold even man to the earth, and enjoys exalted aerial conditions of life. Therefore the marvel is shared equally by learned and unlearned, that the power of flight should have been an endowment of animals sprung from the breed of serpents, or crocodiles, enabling them to move through the air as though they too were of a heaven-born race. The wonder would not be lessened if the animal were a degraded representative of a nobler type, or if it should be demonstrated that even beasts have advanced in the battle of life. The winged reptile, when compared with a bird, is not less astounding than the poetic conceptions in Milton's Paradise Lost of degradation which overtakes life that once was amongst the highest. And on the other hand, from the point of view of the teaching of Darwin in the theories of modern science, we are led to ask whether a flying reptile may not be evidence of the physical exaltation which raises animals in the scale of organisation. The dominance upon the earth of flying reptiles during the great middle period of geological history will long engage the interest of those who can realise the complexity of its structure, or care to unravel the meaning of the procession of animal forms in successive geological ages which preceded the coming of man.

The outer vesture of an animal counts for little in estimating the value of ties which bind orders of animals together, which are included in the larger classes of life. The kindred relationship which makes the snake of the same class as the tortoise is determined by the soft vital organs—brain, heart, lungs—which are the essentials of an animal's existence and control its way of life. The wonder which science weaves into the meaning of the word "reptile," "bird," or "mammal," is partly in exhibiting minor changes of character in those organs and other soft parts, but far more in showing that while they endure unchanged, the hard parts of the skeleton are modified in many ways. For the bones of the reptile orders stretch their affinities in one direction towards the skeletons of salamanders and fishes; and extend them also at the same time in other directions, towards birds and mammals. This mystery we may hope to partly unravel.

CHAPTER II
HOW A REPTILE IS KNOWN

DEFINITION OF REPTILES BY THEIR VITAL ORGANS

The relations of reptiles to other animals may be stated so as to make evident the characters and affinities which bind them together. Early in the nineteenth century naturalists included with the Reptilia the tribe of salamanders and frogs which are named Amphibia. The two groups have been separated from each other because the young of Amphibia pass through a tadpole stage of development. They then breathe by gills, like fishes, taking oxygen from the air which is suspended in water, before lungs are acquired which afterwards enable the animals to take oxygen directly from the air. The amphibian sometimes sheds the gills, and leaves the water to live on land. Sometimes gills and lungs are retained through life in the same individual. This amphibian condition of lung and gill being present at the same time is paralleled by a few fishes which still exist, like the Australian Ceratodus, the lung-fish, an ancient type of fish which belongs to early days in geological time.

This metamorphosis has been held to separate the amphibian type from the reptile because no existing reptile develops gills or undergoes a metamorphosis. Yet the character may not be more important as a ground for classification than the community of gills and lungs in the fish and amphibian is ground for putting them together in one natural group. For although no gills are found in reptiles, birds, or mammals, the embryo of each in an early stage of development appears to possess gill-arches, and gill-clefts between them, through which gills might have been developed, even in the higher vertebrates, if the conditions of life had been favourable to such modification of structure. In their bones Reptiles and Amphibia have much in common. Nearly all true reptiles lay eggs, which are defined like those of birds by comparatively large size, and are contained in shells. This condition is not usual in amphibians or fishes. When hatched the young reptile is completely formed, the image of its parent, and has no need to grow a covering to its skin like some birds, or shed its tail like some tadpoles. The reptile is like the bird in freedom from important changes of form after the egg is hatched; and the only structure shed by both is the little horn upon the nose, with which the embryo breaks the shell and emerges a reptile or a bird, growing to maturity with small subsequent variations in the proportions of the body.

FIG. 1 LUNG OF THE FISH CERATODUS

Partly laid open to show its chambered structure
(After Günther)

THE REPTILE SKIN

Between one class of animals and another the differences in the condition of the skin are more or less distinctive. In a few amphibians there are some bones in the skin on the under side of the body, though the skin is usually naked, and in frogs is said to transmit air to the blood, so as to exercise a respiratory function of a minor kind. This naked condition, so unlike the armoured skin of the true Reptilia, appears to have been paralleled by a number of extinct groups of fossils of the Secondary rocks, such as Ichthyosaurs and Plesiosaurs, which were aquatic, and probably also by some Dinosauria, which were terrestrial.

Living reptiles are usually defended with some kind of protection to the skin. Among snakes and lizards the skin has commonly a covering of overlapping scales, usually of horny or bony texture. The tortoise and turtle tribe shut up the animal in a true box of bone, which is cased with an armour of horny plates. Crocodiles have a thick skin embedding a less continuous coat of mail. Thus the skin of a reptile does not at first suggest anything which might become an organ of flight; and its dermal appendages, or scales, may seem further removed from the feathers which ensure flying powers to the bird than from the naked skin of a frog.

THE REPTILE BRAIN

Although the mode of development of the young and the covering of the skin are conspicuous among important characters by which animals are classified, the brain is an organ of some importance, although of greater weight in the higher Vertebrata than in its lower groups. Reptiles have links in the mode of arrangement of the parts of their brains with fishes and amphibians. The regions of that organ are commonly arranged in pairs of nervous masses, known as (1) the olfactory lobes, (2) the cerebrum, behind which is the minute pineal body, followed by (3) the pair of optic lobes, and hindermost of all (4) the single mass termed the cerebellum. These parts of the brain are extended in longitudinal order, one behind the other in all three groups. The olfactory lobes of the brain in Fishes may be as large as the cerebrum; but among Reptiles and Amphibians they are relatively smaller, and they assume more of the condition found in mammals like the Hare or Mole, being altogether subordinate in size. And the cerebral masses begin to be wider and higher than the other parts of the brain, though they do not extend forward above the olfactory lobes, as is often seen in Mammals. In Crocodiles the cerebral hemispheres have a tendency to a broad circular form. Among Chelonian reptiles that region of the brain is more remarkable for height. Lizards and Ophidians both have this part of the brain somewhat pear-shaped, pointed in front, and elongated. The amphibian brain only differs from the lizard type in degree; and differences between lizards' and amphibian brains are less noticeable than between the other orders of reptiles. The reptilian brain is easily distinguished from that of all other animals by the position and proportions of its regions (see [Fig. 19, p. 53]).

Birds have the parts of the brain formed and arranged in a way that is equally distinctive. The cerebral lobes are relatively large and convex, and deserve the descriptive name "hemispheres." They are always smooth, as among the lower Mammals, and extend backward so as to abut against the hind brain, termed the cerebellum. This junction is brought about in a peculiar way. The cerebral hemispheres in a bird do not extend backward to override the optic lobes, and hide them, as occurs among adult mammals, but they extend back between the optic lobes, so as to force them apart and push them aside, downward and backward, till they extend laterally beyond the junction of the cerebrum with the cerebellum. The brain of a Bird is never reptilian; but in the young Mammal the brain has a very reptilian aspect, because both have their parts primarily arranged in a line. Therefore the brain appears to determine the boundary between bird and reptile exactly.

REPTILIAN BREATHING ORGANS

The breathing organs of Birds and Reptiles which are associated with these different types of brain are not quite the same. The Frog has a cellular lung which, in the details of the minute sacs which branch and cluster at the terminations of the tubes, is not unlike the condition in a Mammal. In a mammal respiration is aided by the bellows-like action of the muscles connected with the ribs, which encase the cavity where the lungs are placed, and this structure is absent in the Frog and its allies. The Frog, on the other hand, has to swallow air in much the same way as man swallows water. The air is similarly grasped by the muscles, and conveyed by them downward to the lungs. Therefore a Frog keeps its mouth shut, and the animal dies from want of air if its mouth is open for a few minutes.

Crocodiles commonly lie in the sun with their mouths widely open. The lungs in both Crocodiles and Turtles are moderately dense, traversed by great bronchial tubes, but do not differ essentially in plan from those of a Frog, though the great branches of the bronchial tubes are stronger, and the air chambers into which the lung is divided are somewhat smaller. The New Zealand Hatteria has the lungs of this cellular type, though rather resembling the amphibian than the Crocodile. The lungs during life in all these animals attain considerable size, the maximum dimensions being found in the terrestrial tortoises, which owe much of their elevated bulk to the dimensions of the air cells which form the lungs.

The lungs of Serpents and Lizards are formed on a different plan. In both those groups of reptiles the dense cellular tissue is limited to the part of the lung which is nearest to the throat. This network of blood vessels and air cells extends about the principal bronchial tube much as in other animals, but as it extends backward the blood vessels become few until the tubular lung appears in its hinder part, as it extends down the body, almost as simple in structure as the air bladder of a fish. Among Serpents only one of these tubular lungs is commonly present, and the structure has a less efficient appearance as a breathing organ than the single lung of the fish Ceratodus ([Fig. 1]). The Chameleons are a group of lizards which differ in many ways from most of their nearest kindred, and the lungs, while conforming in general plan to the lizard type in being dense at the throat, and a tubular bladder in the body, give off on both sides a number of short lateral branches like the fingers of a glove ([Fig. 18, p. 51]).

Thus the breathing organs of reptiles present two or three distinct types which have caused Serpents and Lizards to be associated in one group by most naturalists who have studied their anatomy; while Crocodiles and Chelonians represent a type of lung which is quite different, and in those groups has much in common. These characters of the breathing organs contribute to separate the cold-blooded armoured reptiles from the warm-blooded birds clothed with feathers, as well as from the warm-blooded mammals which suckle their young; for both these higher groups have denser and more elastic spongy lung tissue.

It will be seen hereafter that many birds in the most active development of their breathing organs substantially revert to the condition of the Serpent or Chameleon in a somewhat modified way. Because, instead of having one great bronchial tube expanded to form a vast reservoir of air which can be discharged from the lung in which the reptile has accumulated it, the bird has the lateral branches of the bronchial tubes prolonged so as to pierce the walls of the lung, when its covering membrane expands to form many air cells, which fill much of the cavity of the bird's body (see [Fig. 16]). Thus the bird appears to combine the characters of such a lung as that of a Crocodile, with a condition which has some analogy with the lung of a Chameleon. It is this link of structure of the breathing organs between reptiles and birds that constitutes one of the chief interests of flying reptiles, for they prove to have possessed air cells prolonged from the lungs, which extended into the bones.

CHAPTER III
A REPTILE IS KNOWN BY ITS BONES

Such are a few illustrations of ways in which reptiles resemble other animals, and differ from them, in the organs by means of which the classification of animals is made. But such an idea is incomplete without noticing that the bony framework of the body associated with such vital organs also shows in its chief parts that reptiles are easily recognised by their bones. I will therefore briefly state how reptiles are defined in some regions of the skeleton, for in tracing the history of reptile life the bones are the principal remains of animals preserved in the rocks; and the soft organs which have perished can only be inferred to have been present from the persistence of durable characteristic parts of the skeleton, which are associated with those soft organs in animals which exist at the present day, and are unknown in other animals in which the skeleton is different.

THE HANG OF THE LOWER JAW

The manner in which the lower jaw is connected with the skull yields one of the most easily recognised differences between the great groups of vertebrate animals.

In Mammals.—In every mammal—such as the Dog or Sheep—the lower jaw, which is formed of one bone on each side, joins directly on to the head of the animal, and moves upon a bone of the skull which is named the temporal bone. This character is sufficient to prove, by the law of association of soft and hard parts of the body, that such an animal had warm blood and suckled its young.

FIG. 2

Comparison to show the articulation with the lower jaw in a mammal and Pterodactylus Kochi.
The quadrate bone is lettered Q in this Pterodactyle, and comes between the skull and
the lower jaw like the quadrate bone in a bird and in lizards.

In Birds.—In birds a great difference is found in this region of the head. The temporal bone, which it will be more convenient to name the squamosal bone, from its squamous or scale-like form, is still a part of the brain case, and assists in covering the brain itself, exactly as among mammals. But the lower jaw is now made up of five or six bones. And between the hindermost and the squamosal there is an intervening bar of bone, unknown among mammalia, which moves upon the skull by a joint, just as the lower jaw moves upon it. This movable bone unites with parts of the palate and the face, and is known as the quadrate bone. Its presence proves that the animal possessing it laid eggs, and if the face bones join its outer border just above the lower jaw, it proves that the animal possessed hot blood.

In Reptiles.—All reptiles are also regarded as possessing the quadrate bone. But the squamosal bone with which it always unites is in less close union with the brain case, and never covers the brain itself. Serpents show an extreme divergence in this condition from birds, for the squamosal bone appears to be a loose external plate of bone which rests upon the compact brain case and gives attachment to the quadrate bone which is as free as in a bird. Among Lizards the quadrate bone is usually almost as free. In the other division of existing Reptilia, including Crocodiles, the New Zealand lizard-like reptile Hatteria, called Tuatera, and Turtles, the squamosal and quadrate bones are firmly united with the bones of the brain case, face, and palate, so that the quadrate bone has no movement; and the same condition appears in amphibians, such as Toads and Frogs. With these conditions of the quadrate bone are associated cold blood, terrestrial life, and young developed from eggs.

In Fishes.—Bony fishes, and all others in which separate bones build up the skull, differ from Reptiles and Birds much as those animals differ from Mammals. The union of the lower jaw with the skull becomes complicated by the presence of additional bones. The quadrate bone still forms a pulley articulation upon which the lower jaw works, but between it and the squamosal bone is the characteristic bone of the fish known as the hyomandibular, commonly connected with opercular bones and metapterygoid which intervene, and help to unite the quadrate with the brain case. In the Cartilaginous fishes there is only one bone connecting the jaws with the skull on each side. This appears to prove that just as the structure of the arch of bones suspending the jaw may be complicated by the mysterious process called segmentation, which separates a bone into portions, so simplification and variation may result because the primitive divisions of the material cease to be made which exists before bones are formed.

The principal regions of the skull and skeleton all vary in the chief groups of animals with backbones; so that the Reptile may be recognised among fossils, even in extinct groups of animals and occasionally restored from a fragment, to the aspect which characterised it while it lived.

CHAPTER IV
ANIMALS WHICH FLY

The nature of a reptile is now sufficiently intelligible for something to be said concerning flight, and structures by means of which some animals lift themselves in the air. It is not without interest to remember that, from the earliest periods in human records, representations have been made of animals which were furnished with wings, yet walked upon four feet, and in their typical aspect have the head shaped like that of a bird. They are commonly named Dragons.

FLYING DRAGONS

FIG. 3 From The Battle between Bel and the Dragon

The effigy of the dragon survives to the present day in the figure over which St. George triumphs, on the reverse of the British sovereign. In the luxuriant imaginations of ancient Eastern peoples, dating back to prehistoric ages, perhaps 5000 B.C., the dragons present an astonishing constancy of form. In after-times they underwent a curious evolution, as the conception of Babylon and Egypt is traced through Assyria to Greece. The Wings, which had been associated at first with the fore limb of the typical dragon, become characteristic of the Lion, and of the poet's winged Horse, and finally of the Human figure itself, carved on the great columns of the Greek temples of Ephesus. These flying animals are historically descendants of the same common stock with the dragons of China and Japan, which still preserve the aspect of reptiles. Their interest is chiefly in evidence of a latent spirit of evolution in days too remote for its meaning to be now understood, which has carried the winged forms higher and ever higher in grade of organisation, till their wings ceased to be associated with feelings of terror. The Hebrew cherubim are regarded by H. E. Ryle, Bishop of Exeter, as probably Dragons, and the figure of the conventional angel is the human form of the Dragon.

FIG. 4. FIGURE FROM THE TEMPLE OF EPHESUS

ORGANS OF FLIGHT

Turning from this reference to the realm of mythology to existing nature, the power of flight is popularly associated with all the chief types of vertebrate animals—fishes, frogs, lizards, birds, and mammals. Many of the animals ill deserve the name of flyers, and most are exceptions to different conditions of existence which control their kindred, but it is convenient to examine for a little the nature of the structures by which this movement in the air, which is not always flight, is made possible. Certain fishes, like the lung-fish Ceratodus, of Queensland, and the mud-fish Lepidosiren, are capable of leaving the water and living on land, and for a time breathe air. But neither these fishes nor Periophthalmus, which runs with rapid movement of its fins and carries the body more or less out of water, or the climbing perch, Anabas, carried out of water over the country by Indian jugglers, ever put on the slightest approach to wings.

FLYING FISHES

FIG. 5. THE FLYING FISH EXOCŒTUS

With the fins extended moving through the air

The flight of fishes is a kind of parachute support not unlike that by which a folded paper is made to travel in the air. It is chiefly seen in the numerous species of a genus Exocœtus, allied to the gar-pike (Belone), which is common in tropical seas, and usually from a foot to eighteen inches long. They emerge from the water, and for a time support themselves in the air by means of the greatly developed breast fins, which sometimes extend backward to the tail fin. Although these fins appear to correspond to the fore limbs of other animals, they may not be moved at the will of the fish like the wing of a bird. When the flying fishes are seen in shoals in the vicinity of ships, those fins remain extended, so that the fish is said sometimes to travel 200 yards at a speed of fifteen miles an hour, rising twenty feet or more above the surface of the sea, travelling in a straight line, though sometimes influenced by the wind. Here the organ, which is at once a fin and a wing, consists of a number of thin long rods, or rays, which are connected by membrane, and vary in length to form an outline not unlike the wing of a bird which tapers to a point. The interest of these animals is chiefly in the fact that flight is separated from the condition of having lungs with which it is associated in birds, for although the flying fish has an air bladder, there is no duct to connect it with the throat.

FLYING FROGS

FIG. 6. THE FLYING FROG (RHACOPHORUS)

The membranes of the foot and hand extend between the metatarsal and metacarpal bones, as well as the bones of the digits.

Among amphibians the organs of flight are also of a parachute kind, but of a different nature. They are seen in certain frogs which frequent trees, and are limited to membranes which extend between the diverging digits of the hand and foot, forming webs as fully developed as in the foot of a swimming bird. As these frogs leap, the membranes are expanded and help to support the weight of the body, so that the animal descends more easily as it moves from branch to branch. There is no evidence that the bones of the digits ever became elongated like the fin rays of the flying fish or the wing bones of a Bat; but the web suggests the basis of such a wing, and the possibilities under which wings may first originate, by elongation of the bones of a webbed hand like that of a Flying Frog.

FLYING LIZARDS

The Reptilia in their several orders are remarkable for absence of any modification of the arms which might suggest a capacity for acquiring wings, as being latent in their organisation. Crocodiles, Tortoises, and Serpents are alike of the earth, and not of the air. But among Lizards there are small groups of animals in which a limited capacity for movement through the air is developed. It is best known in the family of small lizards named Dragons, represented typically by the species Draco volans found in the Oriental region of the East Indies and Malay Archipelago.

FIG. 7. THE FLYING DRAGON, DRACO

Forming a parachute by means of the extended ribs

The organ of flight is produced in an unexpected way, by means of the ribs instead of the limbs. The ribs extend outward as far as the arms can stretch, and the first five or six are prolonged beyond the body so as to spread a fold of skin on each side between the arm and the leg. The membrane admits of some movement with the ribs. This arrangement forms a parachute, which enables the animal to move rapidly among branches of trees, extending the structure at will, so that it is used with rapidity too quick to be followed by the eye, as it leaps through considerable distances.

A less singular aid to movement in the air is found in some of the lizards termed Geckos. The so-called Flying Gecko (Platydactylus homalocephalus) has a fringe unconnected with ribs, which extends laterally on the sides of the body and tail, as well as at the back and front of the fore and hind limbs, and between the digits, where the web is sometimes almost as well developed as among Tree Frogs. This is essentially a lateral horizontal frill, extending round the body. Its chief interest is in the circumstance that it includes a membrane which extends between the wrist bones and the shoulder on the front of the arm. That is the only part of the fringe which represents the wing membrane of a bird. The fossil flying reptiles have not only that membrane, but the lateral membranes at the sides of the body and behind the arms.

Other lizards have the skin developed in the direction of the circumference of the body. In the Australian Chlamydosaurus it forms an immense frill round the neck like a mediæval collar. But though such an adornment might break a fall, it could not be regarded as an organ of flight.

FLYING BIRDS

The wings of birds, when they are developed so as to minister to flight, are all made upon one plan; but as examples of the variation which the organs contributing to make the fore limb manifest, I may instance the short swimming limb of the Penguin, the practically useless rudiment of a wing found in the Ostrich or Kiwi, and the fully developed wing of the Pigeon. The wings of birds obtain an extensive surface to support the animal by muscular movements of three modifications of structure. First, the bones of the fore limb are so shaped that they cannot, in existing birds, be applied to the ground for support and be used like the limbs of quadrupeds, and are therefore folded up at the sides of the body, and carried in an unused or useless state so long as the animal hops on the ground or walks, balancing its weight on the hind legs. Secondly, there are two small folds of skin, less conspicuous than those on the arms of Geckos; one is between the wrist bones and the shoulder, and the smaller hinder membrane is between the upper arm and the body. These membranous expansions are insignificant, and would in themselves be inadequate to support the body or materially assist its movements. Thirdly, the bird develops appendages to the skin which are familiarly known as feathers, and the large feathers which make the wing are attached to the skin covering the lower arm bone named the ulna, and the other bones which represent the wrist and hand. The area and form of the bird's wing are due to individual appendages to the skin, which are unknown in any other group of animals. Between the extended wing of the Albatross, measuring eleven feet in spread, and the condition in the Kiwi of New Zealand, in which the wing is vanishing, there is every possible variation in size and form. As a rule, the larger the animal the smaller is the wing area. The problem of the origin of the bird's wing is not to be explained by study of existing animals; for the rowing organ of the Penguin, which in itself would never suggest flight, becomes an organ of flight in other birds by the growth upon it of suitable feathers. Anyone who has seen the birds named Divers feeding under water, swimming rapidly with their wings, might never suspect that they were also organs of aerial flight. The Ostrich is even more interesting, for it has not developed flight, and still retains at the extremities of two of the digits the slender claws of a limb which was originally no wing at all, but the support of a four-footed animal ([Fig. 46, p. 130]).

FIG. 8. POSITION OF BIRDS IN FLIGHT

FLYING MAMMALS

Flight is also developed among mammals. The Insectivora include several interesting examples of animals which are capable of a certain motion through the air. In the tropical forests of the Malay Archipelago are animals known as Flying Squirrels, Flying Opossums, Flying Lemurs, Flying Foxes, in which the skin extends outward laterally from the sides of the body so as to connect the fore limbs with the hind limbs, and is also prolonged backward from the hind limbs to the tail. The four digits are never elongated; the bones of the fore limb are neither longer nor larger than those of the hind limb, and the foot terminates in five little claws as in other four-footed animals. This condition is adapted for the arboreal life which those animals live, leaping from branch to branch, feeding on fruits and leaves, and in some cases upon insects. These mammals may be compared with the Flying Geckos among reptiles in their parachute-like support by extension of the skin, which gives them one of the conditions of support which contribute to constitute flight.

FIG. 9. FLYING SQUIRREL (PTEROMYS)

Bats.—One entire order of mammals—the Bats—not only possess true wings, but are capable of flight which is sustained, and in some cases powerful. The wings are clothed with short hair like the rest of the body, and thus the instrument of flight is unlike that of a bird. The flight of a Bat differs from that of all other animals in being dependent upon a modification of the bones of the fore limb, which, without interfering with the animal's movements as a quadruped, secures an extension of the wing which is not inferior in area to that which the bird obtains by elongation of the bones of the arm and fore-arm and its feathers. The distinctive peculiarity of the Bat's wing is in the circumstance that four of the digits of the hand have their bones prolonged to a length which is often equal to the combined length of the arm and fore-arm. The bones of the digits diverge like the ribs of an umbrella, and between them is the wing membrane, which extends from the sides of the body outward, unites the fore limb with the hind limb, and is prolonged down the tail as in the Flying Foxes. Bats have a small membrane in front of the bones of the arm and fore-arm stretching between the shoulder and the wrist, which corresponds with the wing membrane of a bird; but the remainder of the membranes in Bats' wings are absent in birds, because their function is performed by feathers which give the wing its area. The elongated digits of the Bat's wing are folded together and carried at the sides of the body as though they were a few quill pens attached to its wrist, where the one digit, which is applied to the ground in walking, terminates in a claw.

FIG. 10 NEW ZEALAND BAT FLYING. BARBASTELLE WALKING

The organs which support animals in the air are thus seen to be more or less dissimilar in each of the great groups of animals. They fall into three chief types: first, the parachute; secondly, the wing due to the feathers appended to the skin; and thirdly, the wing formed of membrane, supported by enormous elongation of the small bones of the back of the hand and fingers. The two types of true wings are limited to birds and bats; and no living reptile approximates to developing such an organ of flight as a wing. Judged, therefore, by the method of comparing the anatomical structures of one animal with another, which is termed "comparative anatomy," the existence of flying reptiles might be pronounced impossible. But in the light which the revelations of geology afford, our convictions become tempered with modesty; and we learn that with Nature nothing is impossible in development of animal structure.

CHAPTER V
DISCOVERY OF THE PTERODACTYLE

Late in the eighteenth century, in 1784, a small fossil animal with wings began to be known through the writings of Collini, as found in the white lithographic limestone of Solenhofen in Bavaria, and was regarded by him as a former inhabitant of the sea. The foremost naturalist of the time, the citizen Cuvier—for it was in the days of the French Republic—in 1801, in lucid language, interpreted the animal as a genus of Saurians. That word, so familiar at the present day, was used in the first half of the century to include Lizards and Crocodiles; and described animals akin to reptiles which were manifestly related neither to Serpents nor Turtles. But the term saurian is no longer in favour, and has faded from science, and is interesting only in ancient history of progress. The lizards soon became classed in close alliance with snakes. And the crocodiles, with the Hatteria, were united with chelonians. Most modern naturalists who use the term saurian still make it an equivalent of lizard, or an animal of the lizard kind.

CUVIER

FIG. 11. PTERODACTYLUS LONGIROSTRIS (Cuvier)

The remains are preserved with the neck arched over the back, and the jaws opened upward

Cuvier defined this fossil from Solenhofen as distinguished by the extreme elongation of the fourth digit of the hand, and from that character invented for the animal the name Pterodactyle. He tells us that its flight was not due to prolongation of the ribs, as among the living lizards named Dragons; or to a wing formed without the digits being distinguishable from each other, as among Birds; nor with only one digit free from the wing, as among Bats; but by having the wing supported mainly by a single greatly elongated digit, while all the others are short and terminate in claws. Cuvier described the amazing animal in detail, part by part; and such has been the influence of his clear words and fame as a great anatomist that nearly every writer in after-years, in French and in English, repeated Cuvier's conclusion, maintained to the end, that the animal is a saurian.

FIG. 12. THE SKELETON OF PTERODACTYLUS LONGIROSTRIS

Reconstructed from the scattered bones in Fig. 14, showing the limbs on the left side

Long before fashion determined, as an article of educated belief, that fossil animals exist chiefly to bridge over the gaps between those which still survive, the scientific men of Germany were inclined to see in the Pterodactyle such an intermediate type of life. At first Sömmerring and Wagler would have placed the Pterodactyle between mammals and birds.

GOLDFUSS

FIG. 13. THE PTERODACTYLUS LONGIROSTRIS RESTORED FROM THE REMAINS IN FIG. 11

Showing positions of the wing membranes with the animal at rest

But the accomplished naturalist Goldfuss, who described another fine skeleton of a Pterodactyle in 1831, saw in this flying animal an indication of the course taken by Nature in changing the reptilian organisation to that of birds and mammals. It is the first flash of light on a dark problem, and its brilliance of inference has never been equalled. Its effects were seen when Prince Charles Bonaparte, the eminent ornithologist, in Italy, suggested for the group the name Ornithosauria; when the profound anatomist de Blainville, in France, placed the short-tailed animal in a class between Reptiles and Birds named Pterodactylia; and Andreas Wagner, of Munich, who had more Pterodactyles to judge from than his predecessors, saw in the fossil animal a saurian in transition to a bird.

VON MEYER

But the German interpretation is not uniform, and Hermann von Meyer, the banker-naturalist of Frankfurt a./M., who made himself conversant with all that his predecessors knew, and enlarged knowledge of the Pterodactyles on the most critical facts of structure, continued to regard them as true reptiles, but flying reptiles. Such is the influence of von Meyer that all parts of the world have shown a disposition to reflect his opinions, especially as they practically coincide with the earlier teaching of Cuvier. Owen and Huxley in England, Cope and Marsh in America, Gaudry in France, and Zittel in Germany have all placed the Pterodactyles as flying reptiles. Their judgment is emphatic. But there is weight of competent opinion to endorse the evolutionary teaching of Goldfuss that they rise above reptiles. To form an independent opinion the modern student must examine the animals, weigh their characters bone by bone, familiarise himself, if possible, with some of the rocks in which they are found; to comprehend the conditions under which the fossils are preserved, which have added not a little to the interest in Pterodactyles, and to the difficulty of interpretation.

GEOLOGICAL HISTORY OF PTERODACTYLES IN GERMANY

We may briefly recapitulate the geological history. Those remains of Ornithosaurs which have been mentioned, with a multitude of others which are the glory of the museums of Munich, Stuttgart, Tübingen, Heidelberg, Bonn, Haarlem, and London, have all been found in working the lithographic stone of Bavaria. The whitish yellow limestone forms low, flat-topped hills, now isolated from each other by natural denudation, which has removed the intervening rock. The stone is found at some distance north of the Danube, in a line due north of Augsburg, in the country about Pappenheim, and especially at the villages of Solenhofen, Eichstädt, Kelheim, and Nusplingen. These beds belong to the rocks which are named White Jura limestone in Germany, which is of about the same geological age as the Kimeridge clay in England. Much of it divides into very thin layers, and in these planes of separation the fossils are found. They include the Ammonites lithographicus and a multitude of marine shells, king crabs and other Crustacea, sea-urchins, and other fossils, showing that the deposit was formed in the sea. The preservation of jelly-fish, which so soon disappear when left dry on the beach, shows that the ancient calcareous mud had unusual power of preserving fossils. Into this sea, with its fishes great and small, came land plants from off the land, dragonflies and other insects, tortoises and lizards, Pterodactyles with their flying organs, and birds still clothed with feathers. Sometimes the wing membranes of the flying reptiles are found fully stretched by the wing finger, as in examples to be seen at Munich and in the Yale Museum in Newhaven, in America. At Haarlem there is an example in which the wing membrane appears to be folded much as in the wing of a Bat, when the animal hangs suspended, with the flying membrane bent into a few wide undulations.

The Solenhofen Slate belongs to about the middle period of the history of flying reptiles, for they range through the Secondary epochs of geological time. Remains are recorded in Germany from the Keuper beds at the top of the Trias, which is the bottom division of the Secondary strata; and I believe I have seen fragments of their bones from the somewhat older Muschelkalk of Germany.

THEIR HISTORY IN ENGLAND

In England the remains are found for the first time in the Lower Lias of Lyme Regis, in Dorset, and the Upper Lias of Whitby, in Yorkshire. In Würtemberg they occur on the same horizons. They reappear in England, in every subsequent age, when the conditions of the strata and their fossils give evidence of near proximity to land. In the Stonesfield Slate of Stonesfield, in Oxfordshire, the bones are found isolated, but indicate animals of some size, though not so large as the rare bones of reputed true birds which appear to have left their remains in the same deposit.

At least two Pterodactyles are found in the Oxford clay, known from more or less fragmentary remains or isolated bones; just as they occur in the Kimeridge Clay, Purbeck Limestone, Wealden sandstones, and especially in newer Secondary rocks, named Gault, Upper Greensand, and Chalk, in the south-east of England.

Owing to exceptional facilities for collecting, in consequence of the Cambridge Greensand being excavated for the valuable mineral phosphate of lime it contains, more than a thousand bones are preserved, more or less broken and battered, in the Woodwardian Museum of the University of Cambridge alone. To give some idea of their abundance, it may be stated that they were mostly gathered during two or three years, as a matter of business, by an intelligent foreman of washers of the nodules of phosphate of lime, which, in commerce, are named coprolites. He soon learned to distinguish Pterodactyle bones from other fossils by their texture, and learned the anatomical names of bones from specimens in the University Museum. This workman, Mr. Pond, employed by Mr. William Farren, brought together not only the best of the remains at Cambridge, but most of those in the museums at York and in London, and the thousands of less perfect specimens in public and private collections which passed through the present writer's hands in endeavours to secure for the University useful illustrations of the animal's structure. These fragments, among which there are few entire bones, are valuable, for they have afforded opportunities of examining the articular ends of bones in every aspect, which is not possible when similar organic remains are embedded in rock in their natural connexions.

In England Flying Reptiles disappear with the Chalk. In that period they were widely distributed, being found in Bohemia, in Brazil, and Kansas in the United States, as well as in Kent and other parts of England. They attained their largest dimensions in this period of geological time. One imperfect fragment of a bone from the Laramie rocks of Canada was described, I believe, by Cope, though not identified by him as Ornithosaurian, and is probably newer than other remains.

ASPECT OF PTERODACTYLES

If this series of animals could all be brought together they would vary greatly in aspect and stature, as well as in structure. Some have the head enormously long, in others it is large and deep, characters which are shared by extinct reptiles which do not fly, and to which some birds may approximate; while in a few the head is small and compact, no more conspicuous, relatively, than the head of a Sparrow. The neck may be slender like that of a Heron, or strong like that of an Eagle; the back is always short, and the tail may be inconspicuous, or as long as the back and neck together. These flying reptiles frequently have the proportions of the limbs similar to those of a Bat, with fore legs strong and hind legs relatively small; while in some the limbs are as long, proportionately, and graceful as those of a Deer. With these differences in proportions of the body are associated great differences in the relative length of the wing and spread of the wing membranes.

DIMENSIONS OF THE ANIMALS

The dimensions of the animals have probably varied in all periods of geological time. The smallest, in the Lithographic Slate, are smaller than Sparrows, while associated with them are others in which the drumstick bone of the leg is eight inches long. In the Cambridge Greensand and Chalk imperfect specimens occur, showing that the upper arm bones are larger than those of an Ox. The shaft is one and a half inches in diameter and the ends three inches wide. Such remains may indicate Pterodactyles not inferior in size to the extinct Moas of New Zealand, but with immensely larger heads, animals far larger than birds of flight.

The late Sir Richard Owen, on first seeing these fragmentary remains, said "the flying reptile with outstretched pinions must have appeared like the soaring Roc of Arabian romance, but with the features of leathern wings with crooked claws superinduced, and gaping mouth with threatening teeth." Eventually we shall obtain more exact ideas of their aspect, when the structures of the several regions of the body have been examined. The great dimensions of the stretch of wing, often computed at twenty feet in the larger examples, might lead to expectations of great weight of body, if it were not known that an albatross, with wings spreading eleven feet, only weighs about seventeen pounds.

CHAPTER VI
HOW ANIMALS ARE INTERPRETED BY THEIR BONES

There is only one safe path which the naturalist may follow who would tell the story of the meaning and nature of an extinct type of animal life, and that is to compare it as fully as possible in its several bones, and as a whole, with other animals, especially with those which survive. It is easy to fix the place in nature of living animals and determine their mutual relations to each other, because all the organs—vital as well as locomotive—are available for comparison. On such evidence they are grouped together into the large divisions of Beasts, Birds, and Reptiles; as well as placed in smaller divisions termed Orders, which are based upon less important modifications of fundamental structures. All these characteristic organs have usually disappeared in the fossil. Hence a new method of study of the hard parts of the skeleton, which alone are preserved, is used in the endeavour to discover how the Flying Reptile or other extinct animal is to be classified, and how it acquired its characters or came into existence.

VARIATIONS OF BONES AMONG MAMMALIA

FIG. 14. THE FORE LIMB IN FOUR TYPES OF MAMMALS

Comparison of the fore limb in mammals, showing variation of form of the bones with function

Resemblances and differences in the bones are easily over-estimated in importance as evidence of pedigree relationship. The Mammalia show, by means of such skeletons as are exhibited in any Natural History Museum, how small is the importance to be attached to even the existence of any group of bones in determining its grade of organisation. The whole Whale tribe suckle their young and conform to the distinctive characters in brain and lungs which mark them as being mammals. But if there is one part of the skeleton more than another which distinguishes the Mammalia, it is the girdle of bones at the hips which supports the hind limbs. It is characterised by the bone named the ilium being uniformly directed forward. Yet in the Whale tribe the hip-girdle and the hind limb which it usually supports are so faintly indicated as to be practically lost; while the fore limb becomes a paddle without distinction of digits, and is therefore devoid of hoofs or claws, which are usual terminations of the extremities in mammals. Yet this swimming paddle, with its ill-defined bones—sometimes astonishing in number, as well as in fewness of the finger bones—is represented by the burrowing fore limb of the Mole, which lives underground; by the elongated hoofed legs of the Giraffe, which lives on plains; and the extended arm and finger bones of the Bat, which are equally mammals with the Whale. From such comparison it is seen that no proportion, or form, or length, or use of the bones of the limbs, or even the presence of limbs, is necessarily characteristic of a mammal. No limitation can be placed upon the possible diversity of form or development of bones in unknown animals, when they are considered in the light of such experience of varied structural conditions in living members of a single class.

What is true for the limbs and the bony arches which support them is true for the backbone also, for the ribs, and to some extent for the skull. The neck in the Whale is shortened almost beyond recognition. In the Giraffe the same seven vertebræ are elongated into a marvellous neck; so that in the technical definition of a mammal both are said to have seven neck vertebræ. Yet exceptions show a capacity for variation. One of the Sloths reduces the number to six, while another has nine vertebræ in the neck; proving that there is no necessary difference between a mammal and a reptile when judged by a character which is typically so distinctive of mammals as the number of the neck bones.

The skull varies too, though to a less extent. The Great Ant-eater of South America is a mammal absolutely without teeth. The Porpoises have a simple unvarying row of conical teeth with single roots extending along the jaw. And the dental armature of the jaws, and relative dimensions of the skull bones, exhibit such diversity, in evidence of what may be parted with or acquired, that recognition of the many reptilian structures and bones in the skull of Ornithorhynchus, the Australian Duckbill, demonstrates that the difficulties in recognising an animal by its bones are real, unless we can discover the Animal Type to which the bones belong; and that there is very little in osteology which may not be lost without affecting an animal's grade of organisation.

VARIATION IN SKIN COVERING OF MAMMALS

Even the covering of the body varies in the same class, or even order of animals, so that the familiar growth on the skin is never its only possible covering. The Indian ant-eater, named Manis, which looks like a gigantic fir-cone, the Armadillo, which sheathes the body in rings of bone, bearing only a scanty development of hair, are examples of mammalian hair, as singular as the quills of a Porcupine, the horn of a Rhinoceros, or the growth of hair of varying length and stoutness on different parts of the body in various animals, or the imperfect development of hair in the marine Cetacea. Among living animals it is enough for practical purposes to say that a mammal is clothed with hair, but in a fossil state the hair must usually be lost beyond recognition from its fineness and shortness of growth.

VARIATION IN SKIN COVERING OF BIRDS

No Class of living animals is more homogeneous than Birds; and well-preserved remains prove that, at least as far back in time as the Upper Oolites, birds were clothed with feathers of essentially the same mode of growth and appearance as the feathers of living birds. There may, therefore, be no ground for assuming that the covering was ever different, though some regions of the skin are free from feathers. Yet the variations from fine under-down to the scale-like feathers on the wings of a Penguin, or the great feathers in the wings of birds of flight, or the double quill of the Ostrich group, are calculated to yield dissimilar impressions in a fossil state, even if the fine down would be preserved in any stratum.

VARIATION IN THE BONES OF BIRDS

Osteologically there is less variety in the skeleton of birds than in other great groups of animals. The existing representatives do not exhaust its capability for modification. The few specimens of birds hitherto found in the Secondary strata have rudely removed many differences in the bones which separated living birds from reptiles; so that if only the older fossil birds were known, and the Tertiary and living birds had not existed, a bird might have been defined as an animal having its jaw armed with teeth, instead of devoid of teeth; with vertebræ cupped at both ends, instead of with a saddle-shaped articulation which in front is concave from side from side, and convex from above downwards; in which the bones of the hand are separate, so that three digits terminating in claws can be applied to the ground, instead of the metacarpal bones being united in a solid mass with clawless digits; and in which the tail is elongated like the tail of a lizard. Yet the limits to variation are not to be formulated till Nature has exhausted all her resources in efforts to preserve organic types by adapting them to changed circumstances. Birds may be regarded theoretically as equally capable with mammals of parting with almost every distinctive structure in the skeleton by which it is best known. Even the living frigate bird blends the early joints of the backbone into a compact mass like a sacrum. The Penguin has a cup-and-ball articulation in the early dorsal vertebræ, with the ball in front. And the genus Cypselus has the upper arm bone almost as broad as long, unlike the bird type. Such examples prove that we are apt to accept the predominant structures in an animal type as though they were universal, and forget that inferences based, like those of early investigators, on limited materials may be re-examined with advantage.

VARIATION IN THE BONES OF REPTILES

The true Reptilia, notwithstanding some strong resemblances to Birds in technical characters of the skeleton, display among their surviving representatives an astonishing diversity in the bony framework of the body, exceeding that of the mammalia. This unlooked-for capacity for varying the plan of construction of the skeleton is in harmony with the diversity of structure in groups of extinct animals to which the name reptiles has also been given. The interval in form is so vast between Serpent and Tortoise, and so considerable in structure of the skeleton between these and the several groups of Lizards, Crocodiles, and Hatteria, that any other diversity could not be more surprising. And the inference is reasonable that just as mammals live in the air, in the sea, on the earth, and burrow under the earth, similar modes of existence might be expected for birds and reptiles, though no bird is yet known to have put on the aspect of a fish, and no reptiles have been discovered which roamed in herds like antelopes, or lived in the air like birds or bats, unless these fossil flying animals prove on examination to justify the name by which they are known.

Comparative study of structure in this way demolishes the prejudice, born of experience of the life which now remains on earth, that the ideas of Reptile and of Flight are incongruous, and not to be combined in one animal. The comparative study of the parts of animals does not leave the student in a chaos of possibilities, but teaches us that organic structures, which mark the grades of life, have only a limited scope of change; while Nature flings away every part of the skeleton which is not vital, or changes its form with altering circumstances of existence, enforced by revolutions of the Earth's surface in geological time, in her efforts to save organisms from extinction and pass the grade of life onward to a later age.

The bones are only of value to the naturalist as symbols, inherited or acquired, and vary in value as evidence of the nature and association of those vital organs which differentiate the great groups of the vertebrata.

These distinctive structures, which separate Mammals, Birds, and Reptiles, are sometimes demonstrated by the impress of their existence left on the bones; or sometimes they may be inferred from the characters of the skeleton as a whole.

CHAPTER VII
INTERPRETATION OF PTERODACTYLES BY THEIR SOFT PARTS

THE ORGANS WHICH FIX AN ANIMAL'S PLACE IN NATURE

We shall endeavour to ascertain what marks of its grade of organisation the Pterodactyle has to show. The organs which are capable of modifying the bones are probably limited to the kidneys, the brain, and the organs of respiration. It may be sufficient to examine the latter two.

PNEUMATIC FORAMINA IN PTERODACTYLES

Hermann von Meyer, the historian of the Ornithosaurs of the Lithographic Slate, as early as 1837 described some Pterodactyle bones from the Lias of Franconia, which showed that air was admitted into the interior of the bones by apertures near their extremities, which, from this circumstance, are known as pneumatic foramina. He drew the inference, naturally enough, that such a structure is absolute proof that the Pterodactyle was a flying animal. It was not quite the right form in which the conclusion should have been stated, because the Ostrich and other birds which do not fly have the principal bones pneumatic. Afterwards, in 1859, the larger bones which Professor Sedgwick, of Cambridge, transmitted to Sir Richard Owen established this condition as characteristic of the Flying Reptiles of the Cambridge Greensand. It was thus found as a distinctive structure of the bones both at the beginning and the close of the geological history of these animals. Von Meyer remarks that the supposition readily follows that in the respiratory process there was some similarity between Pterodactyles and Birds. This cautious statement may perhaps be due to the circumstance that in many animals air cavities are developed in the skull without being connected with organs of respiration. It is well known that the bulk of the Elephant's head is due to the brain cavity being protected with an envelope formed of large air cells. Small air cells are seen in the skulls of oxen, pigs, and many other mammals, as well as in the human forehead. The head of a bird like the Owl owes something of its imposing appearance to the way in which its mass is enlarged by the dense covering of air cells in the bones above the brain, like that seen in some Cretaceous Pterodactyles. Nor are the skulls of Crocodiles or Tortoises exceptions to the general rule that an animal's head bones may be pneumatic without implying a pneumatic prolongation of air from the lungs. The mere presence of air cells without specification of the region of the skeleton in which they occur is not remarkable. The holes by which air enters the bones are usually much larger in Pterodactyles than in Birds, but the entrance to the air cell prolonged into the bones is the same in form and position in both groups. So far as can be judged by this character, there is no difference between them. The importance of the comparison can only be appreciated by examining the bones side by side. In the upper arm bone of a bird, on what is known as the ulnar border, near to the shoulder joint, and on the side nearest to it, is the entrance to the air cell in the humerus. In the Pterodactyle the corresponding foramen has the same position, form, and size, and is not one large hole, but a reticulation of small perforations, one beyond another, exactly such as are seen in the entrance to the air cell in the bone of a bird, in which the pneumatic character is found. For it is not every bird of flight which has this pneumatic condition of the bones; and Dr. Crisp stated that quite a number of birds—the Swallow, Martin, Snipe, Canary, Wood-wren and Willow-wren, Whinchat, Glossy-starling, Spotted-fly-catcher, and Black-headed Bunting—have no air in their bones. And it is well known that in many birds, especially water birds, it is only the upper bones of the limbs which are pneumatic, while the smaller bones retain the marrow.

FIG. 15. HEAD OF THE HUMERUS OF THE PTERODACTYLE ORNITHOCHEIRUS

Showing position of the pneumatic foramen on the ulnar side of the bone as in a bird

LUNGS AND AIR CELLS

FIG. 16. LUNGS OF THE BIRD APTERYX PARTLY OPENED ON THE RIGHT-HAND SIDE

The circles are openings of the bronchial tubes on the surface of the lung The notches on the inner edges of the lungs are impressions of the ribs
(After R. Owen)

It may be well to remember that the lungs of a bird are differently conditioned from those of any other animal. Instead of hanging freely suspended in the cone-shaped chamber of the thorax formed by the ribs and sternum, they are firmly fixed on each side, so that the ribs deeply indent them and hold them in place. The lungs have the usual internal structure, being made up of branching cells. The chief peculiarity consists in the way in which the air passes not only into them, but through them. The air tube of the throat of a bird, unlike that of a man, has the organ of voice, not at the upper end in the form of a larynx, but at the lower end, forming what is termed a syrinx. There is no evidence of this in a fossil state, although in a few birds the rings of the trachæa become ossified, and are preserved. But below the syrinx the trachæa divides into two bronchi, tubes which carry the ringed character into the lungs for some distance, and these give off branches termed bronchial tubes, the finer subdivisions from which, in their clustered minute branching sacs, make up the substance of the lung. There is nothing exceptional in that. But towards the outer or middle part of the ventral or under surface of the lungs, four or five rounded openings are seen on each side. Each of these openings resembles the entrance of the air cell into a bone, since it displays several smaller openings which lead to it. Each opening from the lung leads to an air cell. Those cells may be regarded as the blowing out of the membrane which covers the lungs into a film which holds air like a mass of soap bubbles, until the whole cavity of the body of a bird from neck to tail is occupied by sacculated air cells, commonly ten in number, five on each side, though two frequently blend at the base of the neck in the region of the V-shaped bone named the clavicle or furculum, popularly known as the merry-thought. Most people have seen some at least of these semi-transparent bladder-like air cells beneath the skin in the abdominal region of a fowl. The cells have names from their positions, and on each side one is abdominal, two are thoracic, one clavicular, and one cervical, which last is at the base of the neck. The clavicular and abdominal air cells are perhaps the most interesting. The air cell termed clavicular sends a process outward towards the arm, along with the blood vessels which supply the arm. Thus this air cell, entering the region of the axilla or arm-pit, enters the upper arm bone usually on its under side, close to the articular head of the humerus, and in the same way the air may pass from bone to bone through every bone in the fore limb. The hind limbs similarly receive air from the abdominal air cell, which supplies the femur and other bones of the leg, the sacrum, and the tail. But the joints of the backbone in front of the sacrum receive their air from the cervical air sac. The air cells are not limited to the bones, but ramify through the body, and in some cases extend among the muscles. A bird may be said to breathe not only with its lungs, but with its whole body. And it is even affirmed that respiration has been carried on through a broken arm bone when the throat was closed, and the bird under water.

FIG. 17. THE BODY OF AN OSTRICH LAID OPEN TO SHOW THE AIR CELLS WHICH EXTEND THROUGH ITS LENGTH

(After Georges Roché)

Birds differ greatly in the extent to which the aircell system prolonged from the lungs is developed, some having the air absent from every bone, while others, like the Swift, are reputed to have air in every bone of the body.

Comparison shows that in so far as the bones are the same in Bird and Ornithosaur, the evidence of the air cells entering them extends to resemblance, if not coincidence, in every detail. No living group of animals except birds has pneumatic limb bones, in relation to the lungs; so that it is reasonable to conclude that the identical structures in the bones were due to the same cause in both the living and extinct groups of animals. It is impossible to say that the lungs were identical in Birds and Pterodactyles, but so far as evidence goes, there is no ground for supposing them to have been different.

THE LUNGS OF REPTILES

FIG. 18. THE SIDE OF THE BODY OF A CHAMELEON

Ribs removed to show the sacculate branched form of the lung

There is nothing comparable to birds, either in the lungs of living reptiles or in their relation to the bones. The Chameleon is remarkable in that the lung is not a simple bladder prolonged through the whole length of the body cavity, as in a serpent, but it develops a number of large lateral branches visible when the body is laid open. Except near the trachæa, where the tissue has the usual density of a lizard lung, the air cell is scarcely more complicated than the air bladder of a fish, and does not enter into any bone of the skeleton. And although many fishes like the Loach have the swim bladder surrounded by bone connected with the head, it offers no analogy to the pneumatic condition of the bones in the Pterodactyle.

THE FORM OF THE BRAIN CAVITY

But the identity of the pneumatic foramina in Birds and Flying Reptiles is not a character which stands by itself as evidence of organisation, for a mould of the form of the brain case contributes evidence of another structural condition which throws some light on the nature of Ornithosaurs. Among many of the lower animals, such as turtles, the brain does not fill the chamber in the dry skull, in which the same bones are found as are moulded upon the brain in higher animals. For the brain case in such reptiles is commonly an envelope of cartilage, as among certain fishes; and except among serpents, the Ophidia, the bones do not completely close the reptilian brain case in front. The brain fills the brain case completely among birds. A mould from its interior is almost as definite in displaying the several parts of which it is formed as the actual brain would be. And the chief regions of the brain in a bird—cerebrum, optic lobes, cerebellum—show singularly little variation in proportion or position. The essential fact in a bird's brain, which separates it absolutely from all other animals, is that the pair of nerve masses known as the optic lobes are thrust out at the sides, so that the large cerebral hemispheres extend partly over them as they extend between them to abut against the cerebellum. This remarkable condition has no parallel among other vertebrate animals. In Fishes, Amphibians, Reptiles, and Mammals the linear succession of the several parts of the brain is never departed from; and any appearance of variation from it among mammals is more apparent than real, for the linear succession may be seen in the young calf till the cerebral hemispheres grow upward and lop backward, so as to hide the relatively small brain masses which correspond to the optic lobes of reptiles, extending over these corpora-quadrigemina, as they are named, so as to cover more or less of the mass of the cerebellum. From these conditions of the brain and skull, it would not be possible to mistake a mould from the brain case of a bird for that of a reptile, though in some conditions of preservation it is conceivable that the mould of the brain of a bird might be distinguished with difficulty from that of the brain in the lowest mammals. Taken by itself, the avian form of brain in an animal would be as good evidence that its grade of organisation was that of a bird as could be offered.

FIG. 19. THE FORM OF THE BRAIN

THE BRAIN IN SOLENHOFEN PTERODACTYLES

It happens that moulds of the brain of Pterodactyles, more or less complete, are met with of all geological ages—Liassic, Oolitic, and Cretaceous. The Solenhofen Slate is the only deposit in Europe in which Pterodactyle skulls can be said to be fairly numerous. They commonly have the bones so thin as to show the form of the upper surface of the mould of the brain, or the bones have scaled off the mould, or remain in the counterpart slab of stone, so as to lay bare the shape of the brain mass.

In the Museum at Heidelberg a skull of this kind is seen in the long-tailed genus of Pterodactyles named Rhamphorhynchus. It shows the large rounded cerebral hemispheres, which extend in front of cerebral masses of smaller size a little below them in position, which perhaps are as like the brain of a monotreme mammal as a bird.

The short-tailed Pterodactylus described by Cuvier has the cerebral hemispheres very similar to those of a bird, but the relations of the hinder parts of the brain to each other are less clear.

The first specimen to show the back of the brain was found by Mr. John Francis Walker, M.A., in the Cambridge Greensand. I was able to remove the thick covering of cellular bone which originally extended above it, and thus expose evidence that in the mutual relations of the fore and hind parts of the brain bird and ornithosaur were practically identical. Another Cambridge Greensand skull showed that in the genus Ornithocheirus the optic lobes of the brain are developed laterally, as in birds. That skull was isolated and imperfect. But about the same time the late Rev. W. Fox, of Brixton, in the Isle of Wight, obtained from Wealden beds another skull, with jaws, teeth, and the principal bones of the skeleton, which showed that the Wealden Pterodactyle Ornithodesmus had a similar and bird-like brain. In 1888 Mr. E. T. Newton, F.R.S., obtained a skull from the Upper Lias, uncrushed and free from distortion. This made known the natural mould of the brain, which shows the cerebral hemispheres, optic lobes, and cerebellum more distinctly than in the specimens previously known. In some respects it recalls the Heidelberg brain of Rhamphorhynchus in the apparently transverse subdivision of the optic lobes, but it is unmistakably bird-like, and quite unlike any reptile.

IMPORTANCE OF THE BRAIN AND BREATHING ORGANS

So far as the evidence goes, it appears that these fossil flying animals show no substantial differences from birds, either in the mould of the brain or the impress of the breathing organs upon the bones. These approximations to birds of the nervous and respiratory systems, which are beyond question two of the most important of the vital organs of an animal, and distinctive beyond all others of birds, place the naturalist in a singular dilemma. He must elect whether he will trust his interpretation to the soft organs, which among existing animals never vary their type in the great classes of vertebrate animals, and on which the animal is defined as something distinct from its envelope the skeleton and its appendages the limbs, or whether he will ignore them. The answer must choose substantially between belief that the existing order of Nature gives warrant for believing that these vital characteristics which have been discussed might equally coexist with the skeleton of a mammal or a reptile, as with that of a bird, for which there is no particle of evidence in existing life. Or, as an alternative, the fact must be accepted that birds only have such vital organs as are here found, and therefore the skeleton, that may be associated with them, cannot affect the reference of the type to the same division of the animal kingdom as birds. The decision need not be made without further consideration. But brain and breathing organs of the avian type are structures of a different order of stability in most animals from the bones, which vary to a remarkable extent in almost every ordinal group of animals.

TEMPERATURE OF THE BLOOD

The organs of circulation and digestion are necessarily unknown. There are reasons why the blood may have been hot, such as the evidences from the wings of exceptional activity; though the temperature depends more upon the amount of blood in the body than upon the apparatus by which it is distributed. We speak of a Crocodile as cold-blooded, yet it is an animal with a four-chambered heart not incomparable with that of a bird. On the other hand, the Tunny, a sort of giant Mackerel, is a fish with a three-chambered heart, only breathing the air dissolved in water, which has blood as warm as a mammal, its temperature being compared to that of a pig. Several fishes have blood as warm as that of Manis, the scaly ant-eater; and many birds have hotter blood than mammals. The term "hot-blooded," as distinct from "cold-blooded," applied to animals, is relative to the arbitrary human standard of experience, and expresses no more than the circumstance that mammals and birds are warmer animals than reptiles and fishes.

The exceptional temperature of the Flying Fish has led to a vague impression that physical activity and its effect upon the amount of blood which vigour of movement circulates, are more important in raising an animal's temperature than possession of the circulatory organs commonly associated with hot blood, which drive the blood in distinct courses through the body and breathing organs. Yet the kind of heart which is always associated with vital structures such as Pterodactyles are inferred to have possessed from the brain mould and the pneumatic foramina in the bones, is the four-chambered heart of the bird and the mammal. Considering these organs alone—of which the fossil bones yield evidence—we might anticipate, by the law of known association of structures, that nothing distinctly reptilian existed in the other soft part of the vital organisation, because there is no evidence in favour of or against such a possibility.

CHAPTER VIII
THE PLAN OF THE SKELETON

While these animals are incontestably nearer to birds than to any other animals in their plan of organisation, thus far no proof has been found that they are birds, or can be included in the same division of vertebrate life with feathered animals. It is one of the oldest and soundest teachings of Linnæus that a bird is known by its feathers; and the record is a blank as to any covering to the skin in Pterodactyles. There is the strongest probability against feathers having existed such as are known in the Archæopteryx, because every Solenhofen Ornithosaur appears to have the body devoid of visible or preservable covering, while the two birds known from the Solenhofen Slate deposit are well clothed with feathers in perfect preservation. We turn from the skin to the skeleton.

The plan on which the skeleton is constructed remains as evidence of the animal's place in nature, which is capable of affording demonstration on which absolute reliance would have been placed, if the brain and pneumatic foramina had remained undiscovered. With the entire skeleton before us, it is inconceivable that anatomical science should fail to discover the true nature of the animal to which it belonged, by the method of comparing one animal with another. There is no lack of this kind of evidence of Pterodactyles in the three or four scores of skeletons, and thousands of isolated or associated bones, preserved in the public museums of Europe and America.

I may recall the circumstance that the discovery of skeletons of fossil animals has occasionally followed upon the interpretation of a single fragment, from which the animal has been well defined, and sometimes accurately drawn, before it was ever seen. So I propose, before drawing any conclusions from the skeletons in the entirety of their construction, to examine them bone by bone, and region by region, for evidence that will manifest the nature of this brood of Dragons. Their living kindred, and perhaps their extinct allies, assembled as a jury, may be able to determine whether resemblances exist between them, and whether such similarity between the bones as exists is a common inheritance, or is a common acquisition due to similar ways of life, and no evidence of the grade of the organism among vertebrate animals.

The bones of these Ornithosaurs, when found isolated, first have to be separated from the organisms with which they are associated and mixed in the geological strata. This discrimination is accomplished in the first instance by means of the texture of the surface. The density and polish of the bones is even more marked than in the bones of birds, and is usually associated with a peculiar thinness of substance of the bone, which is comparable to the condition in a bird, though usually a little stouter, so that the bones resist crushing better. Pterodactyle bones in many instances are recognised by their straightness and comparatively uniform dimensions, due to the exceptional number of long bones which enter into the structure of the wing as compared with birds. When the bones are unerringly determined as Ornithosaurian, they are placed side by side with all the bones which are most like them, till, judged by the standard of the structures of living animals, the fossil is found to show a composite construction as though it were not one animal but many, while its individual bones often show equally composite characters, as though parts of the corresponding bone in several animals had been cunningly fitted together and moulded into shape.

THE PLAN OF THE HEAD IN ORNITHOSAURS

The head is always the most instructive part of an animal. It is less than an inch long in the small Solenhofen skeleton named Pterodactylus brevirostris, and is said to be three feet nine inches long in the toothless Pterodactyle Ornithostoma from the Chalk of Kansas. Most of these animals have a long, slender, conical form of head, tapering to the point like the beak of a Heron, forming a long triangle when seen from above or from the side. Sometimes the head is depressed in front, with the beak flattened or rounded as in a Duck or Goose, and occasionally in some Wealden and Greensand species the jaws are truncated in front in a massive snout quite unlike any bird. The back of the head is sometimes rounded as among birds, showing a smooth pear-shaped posterior convexity in the region of the brain. Sometimes the back of the head is square and vertical or oblique. Occasionally a great crest of cellular tissue is extended backward from above the brain case over the spines of the neck bones.

There are always from two to four lateral openings in the skull. First, the nostril is nearest to the extremity of the beak. Secondly, the orbits of the eyes are placed far backward. These two openings are always present. The nostril may incline upward. The orbits of the eyes are usually lateral, though their upper borders sometimes closely approximate, as in the woodpecker-like types from the Solenhofen Slate named Pterodactylus Kochi, now separated as another genus. In most genera there is an opening in the side of the head, between the eye hole and the nostril, known as the antorbital vacuity; and another opening, which is variable in size and known as the temporal vacuity, is placed behind the eye. The former is common in the skulls of birds, the latter is absent from all birds and found in many reptiles.

The palate is usually imperfectly seen, but English and American specimens have shown that it has much in common with the palate in birds, though it varies greatly in form of the bones in representatives from the Lias, Oolites, and Cretaceous rocks.

From the scientific aspect the relative size of the head, its form, and the positions and dimensions of its apertures and processes, are of little importance in comparison with its plan of construction, as evidenced by the positions and relations to each other of the bones of which it is formed. There usually is some difficulty in stating the limits of the bones of the skull, because in Pterodactyles, as among birds, they usually blend together, so that in the adult animal the sutures between the bones are commonly obliterated.

Bones have relations to each other and places in the head which can only change as the organs with which they are associated change their positions. No matter what the position of a nostril may be—at the extremity of a long snout, as in an ant-eater, or far back at the top of the head in a porpoise, or at the side of the head in a bird—it is always bordered by substantially the same bones, which vary in length and size with the changing place of the nostril and the form of the head. Every region of the head is defined by this method of construction; so that eye holes and nose holes, brain case and jaw bones, palate and teeth, beak, and back of the skull are all instructive to those who seek out the life-history of these animals. We may briefly examine the head of an Ornithosaurian.

BONES ABOUT THE NOSTRIL

No matter what its form may be, the head of an Ornithosaur always terminates in front in a single bone called the intermaxillary. It sends a bar of bone backward above the visible nostrils, between them; and a bar on each side forms the margin of the jaw in which teeth are implanted. The bone varies in depth, length, sharpness, bluntness, slenderness, and massiveness. As the bone becomes long the jaw is compressed from side to side, and the openings of the nostrils are removed backward to an increasing distance from the extremity of the beak.

The outer and hinder border of the nostril is made by another bone named the maxillary bone, which is usually much shorter than the premaxillary. It contains the hindermost teeth, which rarely differ from those in front, except in sometimes being smaller.

The nasal bones, which always make the upper and hinder border of the nostrils, meet each other above them, in the middle line of the beak.

FIG. 20

Showing that the extremity of the jaws in Rhamphorhynchus was sheathed in horn as in the giant Kingfisher, since the jaws similarly gape in front.
The hyoid bones are below the lower jaw in the Pterodactyle.

The nostrils are unusually large in the Lias genus named Dimorphodon, and small in species of the genus Rhamphorhynchus from Solenhofen. Such differences result from the relative dimensions and proportions of these three bones which margin the nasal vacuity, and by varying growth of their front margins or of their hinder margins govern the form of the snout.

The jaws are most massive in the genera known from the Wealden beds to the Chalk. The palatal surface is commonly flat or convex, and often marked by an elevated median ridge which corresponds to a groove in the lower jaw, though the median ridge sometimes divides the palate into two parallel concave channels. The jaw is margined with teeth which are rarely fewer than ten or more than twenty on each side. They are sharp, compressed from side to side, curved inward, and never have a saw-like edge on the back and front margins. No teeth occur upon the bones of the palate.

In most birds there is a large vacuity in the side of the head between the nostril and the orbit of the eye, partly separated from it by the bone which carries the duct for tears named the lachrymal bone. The same preorbital vacuity is present in all long-tailed Pterodactyles, though it is either less completely defined or absent in the group with short tails. It affords excellent distinctive characters for defining the genera. In the long-tailed genus Scaphognathus from Solenhofen this preorbital opening is much larger than the nostril, while in Dimorphodon these vacuities are of about equal size. Rhamphorhynchus is distinguished by the small size of the antorbital vacuity, which is placed lower than the nostril on the side of the face. The aperture is always imperfectly defined in Pterodactylus, and is a relatively small vacuity compared with the long nostril. In Ptenodracon the antorbital vacuity appears to have no existence separate from the nostril which adjoins the eye hole. And so far as is known at present there is no lateral opening in advance of the eye in the skull in any Ornithosaur from Cretaceous rocks, though the toothless Ornithostoma is the only genus with the skull complete. When a separate antorbital vacuity exists, it is bordered by the maxillary bone in front, and by the malar bone behind. The prefrontal bone is at its upper angle. That bone is known in a separate state in reptiles and, I think, in monotreme mammals. Its identity is soon lost in the mammal, and its function in the skull is different from the corresponding bone in Pterodactyles.

BONES ABOUT THE EYES

FIG. 21. UPPER SURFACE OF SKULL OF THE HERON

Compared with the same aspect of the skull of Rhamphorhynchus

The third opening in the side of the head, counting from before backward, is the orbit of the eye. In this vacuity is often seen the sclerotic circle of overlapping bones formed in the external membrane of the eye, like those in nocturnal birds and some reptiles. The eye hole varies in form from an inverted pear-shape to an oblique or transverse oval, or a nearly circular outline. It is margined by the frontal bone above; the tear bone or lachrymal, and the malar or cheek bone in front; while the bones behind appear to be the quadrato-jugal and post-frontal bones, though the bones about the eye are somewhat differently arranged in different genera.

The eyes were frequently, if not always, in contact with the anterior walls of the brain case, as in many birds, and are always far back in the side of the head. In Dimorphodon they are in front of the articulation of the lower jaw; in Rhamphorhynchus, above that articulation; while in Ornithostoma they are behind the articulation for the jaw. This change is governed by the position of the quadrate bone, which is vertical in the Lias genus, inclined obliquely forward in the fossils from the Oolites, and so much inclined in the Chalk fossil that the small orbit is thrown relatively further back.

Thus far the chief difference in the Pterodactyle skull from that of a bird is in the way in which the malar arch is prolonged backward on each side. It is a slender bar of bone in birds, without contributing ascending processes to border vacuities in the side of the face, while in these fossil animals the lateral openings are partly separated by the ascending processes of these bones. This divergence from birds, in the malar bone entering the orbit of the eye is approximated to among reptiles and mammals, though the conditions, and perhaps the presence of a bone like the post-orbital bone, are paralleled only among Reptiles. The Pterodactyles differ among themselves enough for the head to make a near approach to Reptiles in Dimorphodon, and to Birds in Pterodactylus. In the Ground Hornbill and the Shoebill the lachrymal bones in front of the orbits of the eyes grow down to meet the malar bars without uniting with them. The post-frontal region also is prolonged downward almost as far as the malar bar, as though to show that a bird might have its orbital circle formed in the same way and by the same bones as in Pterodactylus. Cretaceous Ornithosaurs sometimes differ from birds apparently in admitting the quadrato-jugal bone into the orbit. It then becomes an expanded plate, instead of a slender bar as in all birds.

THE TEMPORAL FOSSA

A fourth vacuity is known as the temporal fossa. When the skull of such a mammal as a Rabbit, or Sheep, is seen from above, there is a vacuity behind the orbits for the eyes, which in life is occupied by the muscles which work the lower jaw. It is made by the malar bone extending from the back of the orbit and the process of bone, called the zygomatic process, extending forward from the articulation of the jaw, which arches out to meet the malar bone.

In birds there is no conspicuous temporal fossa, because the malar bar is a slender rod of bone in a line with the lower end of the quadrate bone.

Reptile skulls have sometimes one temporal vacuity on each side, as among tortoises, formed by a single lateral bar. These vacuities, which correspond to those of mammals in position, are seen from the top of the head, as lateral vacuities behind the orbits of the eyes, and are termed superior temporal vacuities. In addition to these there is often in other reptiles a lateral opening behind the eye, termed the inferior temporal vacuity, seen in Crocodiles, in Hatteria, and in Lizards; and in such skulls there are two temporal bars seen in side view, distinguished as superior and inferior. The superior arch always includes the squamosal bone, which is at the back of the single bar in mammals. The lower arch includes the malar bone, which is in front in the single arch of mammals. The circumstance that both these arches are connected with the quadrate bone makes the double temporal arch eminently reptilian.

In Ornithosaurs the lateral temporal vacuity varies from a typically reptilian condition to one which, without becoming avian, approaches the bird type. In skulls from the Lias, Dimorphodon and Campylognathus, there is a close parallel to the living New Zealand reptile Hatteria, in the vertical position of the quadrate bone and in the large size of the vacuity behind and below the eye, which extends nearly the height of the skull. In the species of the genus Pterodactylus, the forward inclination of the quadrate bone recalls the Curlew, Snipe, and other birds. The back of the head is rounded, and the squamosal bone, which appears to enter into the wall of the brain case as in birds and mammals, is produced more outward than in birds, but less than in mammals, so as to contribute a little to the arch which is in the position of the post-frontal bone of reptiles. It is triangular, and stretches from the outer angle of the frontal bone at the back of the orbit to the squamosal behind, where it also meets the quadrate bone. Its third lower branch meets the quadratojugal, which rests upon the front of the quadrate bone, as in Iguanodon, and is unlike Dimorphodon in its connexions. In that genus the supra-temporal bone, or post-orbital bone, appears to rest upon the post-frontal and connect it with the quadrato-jugal. In Dimorphodon the malar bone is entirely removed from the quadrate, but in Pterodactylus it meets its articular end. Between the post-frontal bone above and the quadrato-jugal bone below is a small lunate opening, which represents the lateral temporal vacuity; and so far, this is a reptilian character. But if the thin post-frontal bone were absorbed, Pterodactylus would resemble birds. There is no evidence that the quadrate bone is free in any Ornithosaurs, as it is in all birds, while in Dimorphodon it unites by suture with the squamosal bone. In Ornithostoma the lateral temporal vacuity is little more than a slit between the quadrate bone below, the quadrato-jugal in front, and what may be the post-frontal bone behind (see [Fig. 2, p. 12]).

BONES ABOUT THE BRAIN

The bones containing the brain appear to be the same as form the brain case in birds. The form of the back of the skull varies in two ways. First it may be flat above and flat at the back, when the back of the head appears to be square. This condition is seen in all the long-tailed genera, such as Campylognathus from the Lias and Rhamphorhynchus, and is associated with a high position for the upper temporal bar. Secondly, the back of the head may be rounded convexly, both above and behind. That condition is seen in the short-tailed genera, such as Pterodactylus. But in the large Cretaceous types, such as Ornithocheirus and Ornithostoma, the superior longitudinal ridge which runs back in the middle line of the face becomes elevated and compressed from side to side at the back of the head as a narrow deep crest, prolonged backward over the neck vertebræ for some inches of length. All these three types are paralleled more or less in birds which have the back of the head square like the Heron, or rounded like the Woodpecker; or crested, though the crest of the Cormorant is not quite identical with Ornithocheirus, being a distinct bone at the back of the head in the bird which never blends with the skull. In so far as the crest is reptilian it suggests the remarkable crest of the Chameleon. In the structure of the back of the skull the bones are a modification of the reptilian type of Hatteria in the Lias genus Campylognathus, but the reptilian characters appear to be lost in the less perfectly preserved skulls of Cretaceous genera.

The palate is well known in the chief groups of Ornithosaurs, such as Campylognathus, Scaphognathus, and Cycnorhamphus.

Mr. E. T. Newton, F.R.S., has shown that in the English skull from the Lias of Whitby, the forms of the bones are similar to the palate in birds and unlike the conditions in reptiles. There is one feature, however, which may indicate a resemblance to Dicynodon and other fossil reptiles from South Africa. A slender bone extends from the base of the brain case, named the basi-sphenoid bone, outward and forward to the inner margin of the quadrate bone ([Fig. 22]). A bone is found thus placed in those South African Reptiles, which show many resemblances to the Monotreme and Marsupial Mammals. It is not an ordinary element of the skeleton and is unknown in living animals of any kind in that position. It has been thought possible that it may represent one of the bones which among mammals are diminutive and are included in the internal ear. The resemblance may have some interest hereafter, as helping to show that certain affinities of the Ornithosaurs may lie outside the groups of existing reptiles. Instead of being directed transversely outward, as in the palatal region of Dicynodon lacerticeps, they diverge outward and forward to the inner border of the articulation for the lower jaw which is upon the quadrate bone.

FIG. 22

BONES OF THE PALATE

There is a pair of bones which extend forward from these inner articular borders of the quadrate bones, and converge in a long V-shape till they merge in the hard palate formed by the bones of the front of the beak, named intermaxillary and maxillary bones. The limits of the bones of the palate are not distinct, but there can be no doubt that the front of the V is the bone named vomer, that the palatine bones are at its sides, and that its hinder parts are the pterygoid bones as in birds. There is a long, wide, four-sided, open space in the middle of the palate, between the vomer and the basi-sphenoid bone, unlike anything in birds or other animals.

Professor Marsh, in a figure of the palate in the great skull of the toothless Pterodactyle named Ornithostoma (Pteranodon), from the Chalk of Kansas, found a large oval vacuity in this region of the palate. In that genus the pterygoid bones meet each other between the quadrate bones as in Dicynodon ([Fig. 73, p. 182]). Hence the great palatal vacuity here seen in the Ornithosaur is paralleled by the small vacuity in the South African reptile, which is sometimes distinct and sometimes partly separated from the anterior part of the vacuity which forms the openings of the nostrils on the palate.

The Solenhofen skulls which give any evidence of the palate are exposed in side view only, and the bones, imperfectly seen through the lateral vacuities, are displaced by crushing. They include long strips like the vomerine bones in the Lias fossil, and they diverge in the same way as they extend back to the quadrate bones. The oblique division into vomer in front and pterygoid bone behind is shown by Goldfuss in his original figure of Scaphognathus. Thus there is some reason for believing that all Ornithosaurs have the palate formed upon the same general plan, which is on the whole peculiar to the group, especially in not having the palatal openings of the nares divided in the middle line. It would appear probable that the short-tailed animals have the pterygoid bones meeting in the middle line and triangular; and that they are slender rods entirely separate from each other in the long-tailed genera.

THE TEETH

The teeth are all of pointed, elongated shape, without distinction into the kinds seen in most mammals and named incisors, canines, and grinders. They are organs for grasping, like the teeth of the fish-eating Crocodile of India, and are not unlike the simple teeth of some Porpoises. They are often implanted in oblique oval sockets with raised borders, usually at some distance apart from each other, and have the crown pointed, flattened more on the outer side than on the inner side, usually directed forward and curved inward. As in many extinct animals allied to existing reptiles, the teeth are reproduced by germs, which originate on the inner side of the root and grow till they gradually absorb the substance of the old tooth, forming a new one in its place. Frequently in Solenhofen genera, like Scaphognathus and Pterodactylus, the successional tooth is seen in the jaw on the hinder border of the tooth in use. There is some variation in the character of bluntness or sharpness of the crowns in the different genera, and in their size.

The name Dimorphodon, given to the animal from the Lias of Lyme Regis, expresses the fact that the teeth are of two kinds. In the front of the jaw three or four large long teeth are found in the intermaxillary bone on each side, as in some Plesiosaurs, while the teeth found further back in the maxillary bone are smaller, and directed more vertically downward. This difference is more marked in the lower jaw than in the upper jaw. In Rhamphorhynchus the teeth are all relatively long and large, and directed obliquely forward, but absent from the extremities of the beak, as in the German genus from the Lias named Dorygnathus, in which the bone of the lower jaw (which alone is known) terminates in a compressed spear. In Scaphognathus the teeth are few, more vertical, and do not extend backward so far as in Rhamphorhynchus, but are carried forward to the extremity of the blunt, deep jaw.

In the short-tailed Pterodactyles the teeth are smaller, shorter, wider at the base of the crown, closer together, and do not extend so far backward in the jaw. In Ornithocheirus two teeth always project forward from the front of the jaw. Ornithostoma is toothless.

SUPPOSED HORNY BEAK

Sometimes a horny covering has been suggested for the beak, like that seen in birds or turtles, but no such structure has been preserved, even in the Solenhofen Slate, in which such a structure would seem as likely to be preserved as a wing membrane, though there is one doubtful exception. There are marks of fine blood vessels on some of the jaws, indicating a tough covering to the bone. In Rhamphorhynchus the jaws appear to gape towards their extremities as though the interspace had originally been occupied by organic substance like a horny beak.

LOWER JAW

The lower jaw varies in relative length with the vertical or horizontal position of the quadrate bone in the skull. In Dimorphodon the jaw is as long as the skull; but in the genera from the Oolitic rocks the mandible is somewhat shorter, and in Ornithostoma the discrepancy reaches its maximum. The hinder part of the jaw is never prolonged backward much beyond the articulation, differing in this respect from Crocodiles and Plesiosaurs.

The depth of the jaw varies. It is slender in Pterodactylus, and is probably stronger relatively to the skull in Scaphognathus than in any other form. It fits between the teeth and bones of the alveolar border in the skull, in all the genera. In Dimorphodon its hinder border is partly covered by the descending edge of the malar process which these animals develop in common with some Dinosaurs, and some Anomodont reptiles, and many of the lower mammals. In this hinder region the lower jaw is sometimes perforated, in the same way as in Crocodiles. That condition is observed in Dimorphodon, but is not found in Pterodactylus. The lower jaw is always composite, being formed by several bones, as among reptiles and birds. The teeth are in the dentary bone or bones, and these bones are almost always blended as in most birds and Turtles, and not separate from each other as among Crocodiles, Lizards, and Serpents.

An interesting contour for the lower border of the jaw is seen in Ornithostoma, as made known in figures of American examples by Professors Marsh and Williston. It deepens as it extends backwards for two-thirds its length, stops at an angle, and then the depth diminishes to the articulation with the skull. This angle of the lower jaw is a characteristic feature of the jaws of Mammals. It is seen in the monotreme Echidna, and is characteristic of some Theriodont Reptiles from South Africa, which in many ways resemble Mammals. The character is not seen in the jaws of specimens from the Oolitic rocks, but is developed in the toothed Ornithocheirus from the Cambridge Greensand, and is absent from the jaws of existing reptiles and birds.

FIG. 23. COMPARISON OF THE LOWER JAW IN ECHIDNA AND ORNITHOSTOMA

SUMMARY OF CHARACTERS OF THE HEAD

Taken as a whole, the head differs from other types of animals in a blending of characters which at the present day are found among Birds and Reptiles, with some structures which occur in extinct groups of animals with similar affinities, and perhaps a slight indication of features common to the lowest mammals. It is chiefly upon the head that the diverse views of earlier writers have been based. Cuvier was impressed with the reptilian aspect of the teeth; but in later times discoveries were made of Birds with teeth—Archæopteryx, Ichthyornis, Hesperornis. The teeth are quite reptilian, being not unlike miniature teeth of Mosasaurus. If those birds had been found prior to the discovery of Pterodactyles, the teeth might have been regarded as a link with the more ancient birds, rather than a crucial difference between birds and reptiles.

All the specimens show a lateral temporal hole in the bones behind the eye, and this is found in no bird or mammal, and is typical of such reptiles as Hatteria. The quadrate bone may not be so decisive as Cuvier thought it to be, for its form is not unlike the quadrate of a bird, and different, so far as I have seen, from that of living reptiles. This region of the head is reptilian, and if it occurred in a bird the character would be as astonishing as was the discovery of teeth in extinct birds. These characters of the head are also found in fossil animals named Dinosaurs, in association with many resemblances to birds in their bones.

The palate might conceivably be derived from that of Hatteria by enlarging the small opening in the middle line in that reptile till it extended forward between the vomera; but it is more easily compared with a bird, which the animal resembles in its beak, and in the position of the nares. Excepting certain Lizards, all true existing Reptiles have the nostrils far forward and bordered by two premaxillary bones instead of one intermaxillary, as in Birds and Ornithosaurs. If nothing were known of the animal but its head bones, it would be placed between Reptiles and Birds.

CHAPTER IX
THE BACKBONE, OR VERTEBRAL COLUMN

The backbone is a more deep-seated part of the skeleton than the head. It is more protected by its position, and has less varied functions to perform. Therefore it varies less in distinctive character within the limits of each of the classes of vertebrate animals than either the head or limbs. It is divided into neck bones, the cervical vertebræ; back bones, the dorsal vertebræ; loin bones, the lumbar vertebræ; the sacrum, or sacral vertebræ, which support the hind limbs; and the tail. Of these parts the tail is the least important, though it reaches a length in existing reptiles which sometimes exceeds the whole of the remainder of the body, and includes hundreds of vertebræ. It attains its maximum among serpents and lizards. In frogs it is practically absent. In some of the higher mammals it is a rudiment, which does not extend beyond the soft parts of the body.

THE NECK

The neck is more liable to vary than the back, with the habit of life of the animal. And although mammals almost always preserve the same number of seven bones in the neck, the bones vary in length between the short condition of the porpoise, in which the neck is almost lost, and the long bones which form the neck of the Llama, though even these may be exceeded by some fossil reptiles like Tanystrophœus. In many mammals the neck bones do not differ in length or size from those of the back. In others, like the Horse and Ox, they are much broader and larger.

There is the same sort of variation in the bones of the neck among birds, some being slender like the Heron, others broad like the Swan. But there is also a singular variation in number of vertebral bones in a bird's neck. At fewest there are nine, which equals the exceptionally large number found among mammals in the neck of one of the Sloths. Usually birds have ten to fifteen cervical vertebræ, and in the Swan there are twenty-three. Most of the neck bones of birds are relatively long, and the length of the neck is often greater than the remainder of the vertebral column.

Reptiles usually have short necks. The common Turtle has eight bones in the neck, ten in the back. The two regions are sharply defined by the dorsal shield. Their articular ends are sometimes cupped in front, in the neck, sometimes cupped behind, or convex at both ends, or even flattened, or the articulation may be made exceptionally by the neural arch alone. Nine is the largest number of neck bones in existing Lizards, and there are usually nine in Crocodiles; so that reptiles closely approach mammals in number of the neck bones. It is remarkable that the maximum number in a mammal and in living reptiles should coincide with the minimum number in birds. Therefore the number of cervical vertebræ as an attribute of Mammal, Bird, or Reptile, can only be important from its constancy.

German naturalists affirm on clear evidence that the Solenhofen Pterodactyles have seven cervical vertebræ. In many specimens there can be no doubt about the number, because the neck bones are easily distinguished from those of the back by their size; but the number is not always easy to count.

As in Birds, the first vertebra, or atlas, in Pterodactyles is extremely short, and is generally—if not always—blended with the much longer second vertebra, named the axis. The front of the atlas forms a small rounded cup to articulate with the rounded ball of the basioccipital bone at the back of the skull. The third and fourth vertebræ are longer, but the length visibly shortens in the sixth and seventh.

Sometimes the vertebræ are slender and devoid of strong spinous processes. This is the condition in the little Pterodactylus longirostris and in the comparatively large Cycnorhamphus Fraasii, in which there is a slight median ridge along the upper surface of the arch of the vertebra. This condition is paralleled in birds with long necks, especially wading birds such as the Heron. Other Ornithosaurs, such as Ornithocheirus from the Cretaceous rocks, have the neck much more massive. The vertebræ are flattened on the under side. The arch above the nervous matter of the spinal cord has a more or less considerable transverse expansion, and may even be as wide as long. These vertebræ have proportions and form such as may be seen in Vultures or in the Swan. In either case the form of the neck bones is more or less bird-like, and the neural spine may be elevated, especially in Pterodactyles with long tails.

One of the most distinctive features of the neck bones of a bird is the way in which the cervical ribs are blended with the vertebræ. They are small, and each is often prolonged in a needle-like rod at the side of the neck bone.

In Ornithocheirus the cervical rib similarly blends with the vertebra by two articulations, as in mammals, so that it might escape notice but for the channel of a blood vessel which is thus inclosed. In several of the older Pterodactyles from Solenhofen the ribs of the neck vertebræ remain separated, as in a Crocodile, though still bird-like in their form, anterior position, and mode of attachment. In Terrapins and Tortoises the long neck vertebræ have no cervical ribs.

FIG. 24 UNITED ATLAS AND AXIS OF ORNITHOCHEIRUS