The order Squamata, so called because of the dermal covering of overlapping horny scales, comprises the great majority of living reptiles. Although the scaly covering is characteristic of nearly all the members of the order, the most essential differences distinguishing them from other reptiles are, as usual, found in the skeleton, and especially in the skull. The quadrate bone, that to which the lower jaw is articulated on each side, is not wedged in immovably between other bones of the skull, as in all other reptiles, but is, instead, freely articulated with the cranium in such a way that its lower end moves both backward and forward, as well as inward and outward. This freedom of movement has in the past been thought to be due to the loss of a lower temporal arch, a bony bar connecting the lower end of the quadrate with the hind end of the upper jaw, which is very characteristic, for instance, of the Rhynchocephalia. Indeed, because of the many primitive characters which the lizards possess, it has generally been supposed that the order was an early branch of the rhynchocephalian stem. But we are now quite sure that the lizards are as primitive as the Rhynchocephalia, and that their origin, as an independent branch of the reptilian stem, goes quite as far if not farther back—quite sure that the ancestors of the lizards never had a lower temporal arcade and two temporal vacuities, but that the looseness of the quadrate bone has been due to the gradual loss of a bone which covered the whole side of the skull until only the upper part of it was left. In other words, the ancestral skull of the Squamata must have been like that of Araeoscelis, more fully described under the Protorosauria, a group than which there is perhaps none more closely allied to the Squamata.

The bones of the roof of the mouth of the Squamata—that is, of the palate—are narrow and long, and are not closely articulated, as in most other reptiles; they often bear teeth, a primitive character. The teeth of all living lizards and snakes are not inserted in sockets, as are those of the crocodiles, but are co-ossified to the margins or sides of the jaws or the bones of the palate. But this is probably not a primitive character; doubtless the teeth of the early lizards were inserted in sockets like those of most other reptiles. The shoulder bones are absent in many and vestigial in some others. When present and fully developed, they comprise the shoulder-blades or scapulae, a single coracoid on each side, the clavicles, and an interclavicle. The vertebrae, except in some lizards, are procoelous, that is, with the body concave in front and convex behind, a peculiar structure that was developed only in crawling animals. In addition to the usual articulations for the union of the vertebrae there are also, in some of the lizards and mosasaurs and all of the snakes, additional ones called the zygosphene and zygantrum, which will be best understood by reference to Fig. 12, p. 28. But little less characteristic than the loose articulation of the lower jaws, so unique in this order of reptiles, is the manner of attachment of the ribs. They are always single-headed, articulating only with the body or lower part of the vertebra. The single-headed ribs of the plesiosaurs articulate with a projection on each side of the arch of the vertebra; those of the turtles to the space between the adjacent vertebrae; nearly all other reptiles have double-headed ribs, articulating in various ways. This character, it is seen, though apparently a simple one, immediately distinguishes a lizard or a snake from all other animals, except the thalattosaurs and protorosaurs.

There is much difference of opinion among naturalists as to the proper classification of the different groups of this order of reptiles. Usually it is divided into four suborders, the Lacertilia or lizards; the Dolichosauria or long-necked lizards of the past; the Mosasauria, or extinct swimming lizards; and the Serpentes or Ophidia, the snakes. It matters very little which classification one accepts so long as it is remembered that the first three groups are closely related to each other.

LIZARDS

Popularly a lizard is any four-legged reptile covered with scales, but such a definition is not strictly correct, since some lizards are legless and some other four-legged reptiles are covered with horny scales, notably the tuatera or Sphenodon of New Zealand, a reptile long classed with lizards, but now known to belong to quite a different order. Bearing in mind those characters given as characters of the order, it will be necessary to mention only those distinguishing the lizards from the snakes.

It is true that the great majority of lizards have four legs, while the snakes are always functionally legless, but there are some lizards, like the glass snakes and the amphisbaenas, or slow lizards, which are quite legless and there are some snakes which have small but functionless hind legs. As usual, more important differences are found in the skull. The brain-case in all snakes is surrounded on all sides by bone, for the better protection of the brain, with the head resting quite prone on the ground. The brain of the lizards, for the most part, is protected on the sides and in front by a simple membrane. Nearly all lizards have movable eyelids, while snakes do not; snakes have a single lung, and a protrusible tongue, which very few lizards possess; and the lower jaws in front are united in the snakes by a ligament only. Notwithstanding these differences, the snakes and lizards are closely related animals, and must have come from a common ancestry; among all reptiles the known geological history of the snakes is shortest.

Lizards, on the other hand, have a very high antiquity, beginning, as we now know, at least as long ago as early Triassic times. They still have many primitive characters in their structure and are the least advanced type of reptiles now living, with the exception of the tuatera. Their remains are seldom found in the rocks, probably because they have always been so strictly terrestrial in habit, for the most part seldom frequenting even the vicinity of the water. The true lizards now living number about eighteen hundred kinds, classified into about twenty families, divided among four chief groups, of which the chameleons, the amphisbaenas, our common lizards, and the monitors are representatives.

Most living lizards are inhabitants of warm climates, though some extend rather far north in the temperate zone. With the exception of New Zealand, and the polar and subpolar regions, lizards are found in all parts of the world. The great majority live only in high and dry places, though some are denizens of low and marshy places, a few even not being averse to the water. They are, for the most part, spry in their movements, some little ones scarcely six inches in length taxing a vigorous man’s speed to capture; and many are expert climbers of cliffs, trees, and even the ceilings of residences. Some, the remarkable little flying dragons of Ceylon, have an extraordinary development of the skin on the sides of the body, supported by the expanded ribs, forming a sort of parachute whereby the creatures can sail considerable distances through the air. Nearly all are carnivorous, feeding upon small mammals, birds, other reptiles, frogs, and insects; a few only are herbivorous, such as the iguanas, which are often used for human food. Nearly all lizards are oviparous, laying from two to thirty eggs. In size the great majority are small, less than a foot in length; but some, such as the monitors and iguanas, reach a length of from four to six feet, or even more, and certain extinct monitors of India are known to have attained a length of thirty feet. They are, for the most part, slender, graceful, prettily marked, and quite inoffensive creatures. A few are short, flat, or stumpy in shape, such as the so-called horned toad. One or two species only, the “Gila monsters,” are reputed to be venomous.

There is but a single species of lizard now living which is in any true sense aquatic in habit, the well-known sea-lizard of the Galapagos Islands, scientifically known as Amblyrhynchus cristatus. It is a large lizard, with a short rounded head, a flat tail, and webbed feet. Its specific name is derived from the erect fringed crest along its back and tail. Its habits are best given in Darwin’s words:

It is extremely common on all the islands throughout the group, and lives exclusively on the rocky sea-beaches, being never found, at least I never saw one, even ten yards inshore. It is a hideous looking creature, of a dirty black color, stupid and sluggish in its movements. The usual length of a full grown one is about a yard, but there are some even four feet in length; a large one weighed twenty pounds. The tails are flattened sideways, and all four feet are partially webbed. They are occasionally seen some hundred yards from the shore swimming about. When in the water this lizard swims with perfect ease and quickness, by a serpentine movement of the body and flattened tail—the legs being motionless and closely collapsed to the sides. A seaman on board sank one, with a heavy weight attached to it, thinking thus to kill it directly; but when an hour afterward he drew up the line it was quite active. Their limbs and strong claws are admirably adapted for crawling over the rugged and fissured masses of lava, which everywhere forms the coast. The nature of this lizard’s food (seaweed) as well as the structure of the tail and feet, and the fact of its having been seen voluntarily swimming out at sea, absolutely proves its aquatic habits; yet there is in this respect one strange anomaly, namely, that when frightened it will not leave the island. Hence it is easy to drive these lizards down to any little point overhanging the sea, where they will sooner allow a person to catch hold of their tails than jump into the water. They do not seem to have any notion of biting; but when much frightened they squirt a drop of fluid from each nostril. I threw one several times as far as I could into a deep pool left by the retreating tide, but it invariably returned by a direct line to the spot where I stood. It swam near the bottom, with a very graceful and rapid movement, and occasionally aided itself over the uneven ground with its feet. As soon as it arrived near the edge, but still being under water, it tried to conceal itself under the tufts of seaweed, or it entered some crevice. I several times caught the same lizard by driving it to a point, and, though possessed of such perfect powers of diving and swimming, nothing could induce it to enter the water; and as often as I threw it in it returned in the manner described above. Perhaps this singular piece of apparent stupidity may be accounted for by the circumstance that this reptile has no enemies whatever on shore, whereas at sea it must often fall a prey to the numerous sharks.

These lizards are of much interest as indicating one of the ways in which true land reptiles have become aquatic in their habits. Tempted by the abundance of food growing in shallow water a little beyond their reach, the reptiles ventured farther and farther to obtain it. The tail gradually became a propelling organ, though the lizard still retained in large measure its land habits and land feet, because of the dangers from its water enemies. It is not at all improbable that, in course of time, were these Galapagos lizards left unmolested, they would become fleeter swimmers by the development of a terminal caudal fin and paddle-like legs, thus competing with their aquatic enemies and no longer needing recourse to the land for protection. They also serve to indicate that long-tailed aquatic reptiles never used their legs to an appreciable extent as organs of propulsion in the water.

Flat-headed lizards.—Among the living lizards there is one group, called the monitors, which have so many characters peculiar to themselves that they seem rightfully entitled to an isolated place among the lizards of the present time. The group includes about thirty species, all belonging in the one genus Varanus, and all living in India, Africa, and Australia. In size, some of the species of Varanus are the largest of all terrestrial lizards known in the past or present; in other ways also they have reached the maximum of specialization among lizards. The head is pointed, broad, and flat, and the body and tail are long. They have nine vertebrae in the neck, a larger number than is to be found in any other terrestrial lizard. Unlike other lizards they have a protrusible tongue like that of the snakes. All are carnivorous in habit, feeding upon small backboned animals, insects, and especially upon eggs, which they crush between their teeth while holding them aloft. Most species live wholly upon the land, and some are arboreal. Others, especially those of the Nile, live about water and are excellent swimmers. The terrestrial species have a round tail and small external nostrils, but the water species have the tail much flattened, and the nostrils have large cavities, which, when closed under water, are said to serve as reservoirs of air for respiration. Of one of these swimming species Annandale writes:

Varanus salvator is common in Lower Siam where it is equally at home on land, in water, and among the branches of trees. The eggs are laid in hollow tree trunks. When in the water the lizard swims beneath the surface, the legs being closely applied to the sides, and the tail functioning both as oar and rudder.

These lizards take to the water to escape from their land enemies and not for food, a habit also known among certain other lizards, and one precisely the reverse of that of the Galapagos lizards. It would seem very probable that animals of such carnivorous habits as are the monitors might easily learn to capture water animals for food and thus eventually become aquatic in habit. This inclination toward, and partial adaptation to, water habits in the monitors is of much interest because in all probability the instinct is one of long inheritance from those remote ancestors which gave origin to the truly aquatic members of the order. Though the known geological history of the monitors does not extend far back, they are so intimately allied in their anatomical structure to the aquatic and semiaquatic lizards of Cretaceous times that there could seem to be no doubt of the common ancestry.

Dolichosaurs.—About fifty years ago Professor Owen, the famous English paleontologist, described a peculiar semiaquatic lizard from the Cretaceous rocks of England to which he gave the name Dolichosaurus, in allusion to the slender form of the body. Just what relations these slender lizards have to modern lizards has long been a problem; some have thought that they were their progenitors, but there are very good reasons for doubting this. No modern lizards, save the monitors, have more than eight vertebrae in the neck, while these dolichosaurs had as many as seventeen, a remarkable specialization for aquatic life that could hardly have been lost by their descendants. For this reason the dolichosaurs have usually been considered as representing a distinct suborder. But they have many resemblances otherwise to the monitors. They were semiaquatic in habit, and never more than six feet in length. They are yet imperfectly known, and no restoration of any form has hitherto been attempted. Their peculiar interest lies in the elongation of the neck, quite like that of the wholly unrelated nothosaurs and proganosaurs, which have been described in the foregoing pages. Doubtless similar habits in each had like results, but just what these habits were in the slender lizards we do not yet know.

Aigialosaurs.—Within recent years a number of other lizards have been made known from the Lower Cretaceous rocks of Dalmatia which present most remarkable intermediate characters between the monitors, dolichosaurs, and the mosasaurs, the famous sea-lizards of Upper Cretaceous age. Some of these lizards had twelve or thirteen vertebrae in the neck, while others had but seven—an unusually short neck characteristic of the mosasaurs. These latter kinds, belonging to two or three genera, are included in a distinct group. They were long and slender, the head long and pointed. The teeth, conical and sharp, were attached in shallow pits, quite as in the mosasaurs. The lower jaws had a hinge just back of the teeth, as in the mosasaurs, of which the only trace in modern lizards is found among the monitors. Still more remarkable, though perhaps not so easily appreciated, is the shape of the quadrate bone, with a broad flaring rim for the ear cavity, quite unlike that of land lizards, but quite like that of the mosasaurs. In fact, the very peculiar skull is almost identical with that of the true sea-lizards. The body and tail also resemble those of the mosasaurs more than those of the monitors, but there is a firm attachment of the pelvis to the backbone, and the legs are long and lizard-like, though not as long as those of land lizards. The feet were webbed in life, and the toes have no claws, conclusively demonstrating their water habits. The vertebrae indeed have the same peculiar articulations, called zygosphenes, as in most of the mosasaurs. The largest aigialosaurs were about six feet in length, that is, of about the size of the smallest known mosasaurs.

We have then in the aigialosaurs nearly every known intermediate character that we could wish for in a connecting link between the mosasaurs and the monitors, lizards that were equally at home on land or in the water, and there can be scarcely a doubt that they were either the direct ancestors or closely akin to the direct ancestors of the strictly marine mosasaurs; and scarcely a doubt that they were the descendants of the actual forbears of the modern monitors, which, as we have seen, have acquired partial aquatic habits in escaping from their enemies. The dolichosaurs we can now understand were a side branch from these semiaquatic aigialosaurs which, specializing in another direction, quickly came to grief, perhaps in competition with their more agile and skilful short-necked kin.

Taking all these facts into consideration it seems best to unite the monitors, dolichosaurs, and aigialosaurs into one group of the Lacertilia, the Platynota, intermediate in place between the true land lizards and the truly aquatic mosasaurs.

MOSASAURS

At St. Pietersberg, a small mountain in the vicinity of Maestricht, Holland, there are immense subterranean stone quarries, which have been worked for more than a thousand years. The stone quarried from them is a sandy limestone of Upper Cretaceous age containing many well-preserved remains of extinct animals that have long been sought by collectors of fossils. In 1776 Major Drouin—an officer of a near-by garrison, one of much military importance in those days—secured from one of these quarries some bones of an extinct reptile, which, though of interest, afforded but little information concerning the structure and affinities of the animal to which they had once belonged. In 1780 a very perfect skull, in excellent preservation, of the same kind of an animal was obtained from the same quarry by Dr. Hofmann, an army surgeon of the same garrison, whose interest in such things had been incited by Major Drouin’s collections. This specimen, so renowned in science, has had a remarkable and eventful human history, in part related by St. Faujas de Fond, a French commissary of the “Army of the North,” and one of the participants:

In one of the great galleries or subterranean quarries in which the Cretaceous stone of St. Pieter’s Mount is worked, about five hundred paces from the entrance, and ninety feet below the surface, the quarrymen exposed part of the skull of a large animal in a block of stone which they were engaged in quarrying. On discovering it they suspended their work and went to inform Dr. Hofmann, surgeon to the forces at Maestricht, who for some years had been collecting the fossils from the quarry, remunerating the workmen liberally for the discovery and preservation of them. Dr. Hofmann, arriving at the spot, saw with extreme pleasure the indication of a magnificent specimen; he directed the operations of the men, so that they worked out the block without injury to the fossil, and he then, by degrees, cleared away the yielding matrix and exposed the extraordinary jaws and teeth, which have since been the subject of so many drawings, descriptions, and discussions. This fine specimen which Dr. Hofmann had transported with so much satisfaction to his collection, soon became, however, a source of much chagrin to him. Dr. Goddin, one of the canons of Maestricht, who owned the surface of the soil beneath which was the quarry whence the fossil was obtained, when the fame of the fossil reached his ears, pleaded certain feudal rights in support of his claim to it. Hofmann resisted and the canon went to law. The whole chapter supported their reverend brother, and the decree ultimately went against the poor surgeon, who lost both the specimen and his money, for he was made to pay the costs of the action. The canon, leaving all remorse to the judges who pronounced the iniquitous sentence, became the happy and contented possessor of this unique example of its kind.

[Translation by Leidy.]

But the canon was ultimately despoiled of his ill-gotten treasure. At the siege of Maestricht in 1795, the famous skull to which Hofmann had devoted so much anxious thought and labor, fell into the hands of the French and was carried off as one of the spoils of war. So widely celebrated had the specimen become during the fifteen years which had elapsed since its discovery, through the writings of several noted scientific men, that the French general commanded his artillerists to spare the house in which it was known to be. The canon, however, shrewdly suspecting that such an unexpected and extraordinary mark of favor was not for his own sake but rather for the sake of the famous fossil, had it removed and carefully hidden in a house in the city. After the capitulation of Maestricht the eagerly sought-for fossil was not to be found, and the offer of a reward of six hundred bottles of wine, so the story goes, was made for its recovery. So tempting was the offer that, ere long, it was brought in triumph to the house of St. Faujas de Fond, by a half-dozen grenadiers, whence it was later transferred to Paris, where it now is.

We may well sympathize with Dr. Hofmann in the loss of his cherished specimen, since, had it not been for his zeal, money, and labor, it would never have escaped the usual fate of such things—complete destruction. But we must remember that St. Faujas de Fond, the recorder of this history, was a Frenchman, and somewhat interested in robbing the reverend canon of it; possibly there is another side of the story which has never been told.

After peace was declared, one has regretfully to add that the canon, not Dr. Hofmann, was reimbursed for it, or so it is said. Cuvier rather naïvely says that it was ceded to the Garden of Plants of Paris, perhaps in the way that many other things are ceded to the conqueror in time of war. The specimen is really a good one, even when compared with many found in recent years, and there is little wonder that the cupidity of St. Fond was incited by it. Casts of it are now or have been in nearly every noted museum of the world, and pictures of it illustrated nearly every textbook of geology published during the first three-quarters of the past century. It had been the subject of considerable controversy even before it came into the hands of Cuvier. Peter Camper figured and described the skull as that of a whale or “breathing fish”; while St. Fond himself later called it a crocodile. Crocodile or alligator skeletons were rare in those days, and St. Fond made a special trip to the British Museum to study one. But it was really Adrian Camper, a son of Peter Camper, who deserves the credit, so often wrongly ascribed to Cuvier, for the recognition of the true nature of the fossil. He insisted that the animal was a lizard allied to the living monitors, an opinion which it will be seen has finally been proved to be correct within very recent years.

In 1808 this famous skull, and all other known remains of a similar nature, came under the observation of Cuvier, the renowned French naturalist and paleontologist, who confirmed the views of Adrian Camper. He fully described and figured all the known parts of the skeleton that had later come to light, calling the animal the great lizard of the Meuse, the river near which Hofmann’s specimen was found. Conybeare, a well-known paleontologist of England, some years later formally christened it Mosasaurus, a transliteration of Cuvier’s phrase, from the Latin Mosa, for Meuse, and saurus, a lizard. For more than half a century Cuvier’s figure of the skull of the original specimen appeared in works on geology over the name Mosasaurus hofmanni, or Mosasaurus camperi. One could wish that the former name for the species might prevail, in recognition of the zealous doctor who was so shabbily treated in his possession of the specimen.

For some years the few specimens discovered by Drouin and Hofmann were all that were known of the mosasaurs. A few others of related forms were discovered in England, and some were reported from New Jersey by early explorers, but there was little published about the mosasaurs till 1843, when Dr. August Goldfuss, a noted German paleontologist, described and beautifully figured an excellent specimen from the United States. This specimen also had a rather eventful history. It was discovered early in the fourth decade by Major O’Fallen, an Indian agent, near the Great Bend of the Missouri River, whence it was transported by him to St. Louis and placed in his garden as a curiosity. It happened that Prince Maximilian of Wied, the famous naturalist, in his travels through the United States, saw the specimen and secured it, taking it to Germany on his return. He presented it to the Museum of Haarlem where Goldfuss saw and described it. Rather oddly, this specimen was of a species closely allied to the original one of Maestricht, a species which has since only rarely been found. It was called Mosasaurus maximiliani by Goldfuss, though some time previously, it has since been found, some fragments of the same species were described by Harlan, an American author, under the name Ichthyosaurus missouriensis. Goldfuss’ paper was strangely overlooked by subsequent writers, and it was not till the discovery of numerous remains of mosasaurs by Leidy, Cope, and Marsh in the chalk of western Kansas, nearly thirty years later, that much was added to the world’s knowledge of these strange reptiles.

Perhaps nowhere in the world are the fossil remains of marine animals more abundantly and better preserved than in these famous chalk deposits of Kansas. The exposures are of great extent—hundreds of square miles—and the fossil treasures they contain seem inexhaustible. Long-continued explorations by collectors have brought to light thousands of specimens of these swimming lizards, some of them of extraordinary completeness and perfect preservation, so complete and so perfect that there is scarcely anything concerning the mosasaurs which one might hope to learn from their fossil remains that has not been yielded up by these many specimens. The complete structure and relations of all parts of the skeleton, impressions of the bodies made in the soft sediments before decomposition had occurred, the character of their food, the nature of the skin covering, and even some of the color markings of the living animals have all been determined with certainty. Not only from Kansas, but also from many other parts of the world, have remains of these animals been discovered, until now it may truthfully be said that no other group of extinct reptiles is better represented by known fossil remains than the mosasaurs. From England, Belgium, Russia, and France in Europe; from New Jersey, Georgia, Alabama, Mississippi, Texas, New Mexico, Colorado, Kansas, Nebraska, the Dakotas, Wyoming, and other places in the United States; from New Zealand and South America they have been obtained in greater or less abundance and perfection.

Their geological history is relatively brief, notwithstanding their wide distribution over the earth in such great numbers and diversity. The earliest are known from near the beginning of the Upper Cretaceous of New Zealand, whence it is believed by some that they migrated to other parts of the world, appearing in North America some time later. They reached their culmination in size, numbers, and variety very soon, and then disappeared forever before the close of Cretaceous time. The largest complete specimen of a mosasaur known measures a little more than thirty feet in length, but incomplete skeletons of others indicate a maximum length of about forty feet. The skulls of the largest species are about five feet long. The smallest known adult skeletons are scarcely eight feet in length. There are now known at the present time seven or eight genera of three distinct types, all belonging to one family, the Mosasauridae, including about twenty-five known species. While a few of the genera are widely distributed over the earth, the species are all of restricted range, indicating, perhaps, non-migratory habits.

The adaptation of the mosasaurs to an aquatic life was very complete, though perhaps not so complete as was that of the ichthyosaurs. The skull is flattened, narrow, and more or less elongate, but large in proportion to the remainder of the skeleton—nearly one-sixth of the entire length; that relative size doubtless is indicative of very predaceous and pugnacious habits. The teeth in the typical forms are numerous, strong, and sharp, conical in shape, and recurved. Not only are there numerous teeth in both the upper and lower jaws, but there are also two rows of strong teeth implanted in the back part of the palate, upon bones called pterygoids, the use of which will be understood later. The teeth were inserted on large, tumid, bony bases, rather loosely attached in shallow pits or alveoli, unlike the teeth of all modern lizards. Such a mode of attachment of the teeth doubtless had some relation to the habits of the animals concerning which we are not quite clear. They were easily dislodged, and, in consequence, of very unequal size, some full grown, some small, and others just appearing above the surface of the gums in the living animals. The frequent loss of teeth and their constant and easy replacement by new ones is a peculiarity of predaceous reptiles, thereby insuring their best functional use.

The external nostrils, of large size, were situated at a considerable distance back of the end of the snout, but not nearly so far back or so near the eyes as were the nostrils of the ichthyosaurs, plesiosaurs, and phytosaurs. Their size and position suggest a use like that of the modern aquatic monitors, as mentioned on a preceding page. The eyes were of moderate size, those of the less purely aquatic forms being directed more laterally than those of species of more distinctly diving habits. They were protected by a stout ring of bony plates, as were the eyes of all truly aquatic reptiles of the past. The ears, also, in most if not all mosasaurs, had a thick cartilaginous ear-drum in place of a simple membrane, evidently, as Dollo has shown, for better protection under undue pressure of the water in deep diving.

As in all other lizards, the bones with which the lower jaws articulate, the quadrates, were loosely attached at the upper end, permitting great freedom of movement in all directions, more even than the land lizards have. The lower jaws were long and powerful, armed with a single row of teeth on each side, from sixteen to eighteen in number. Just back of the teeth, a little beyond the middle, each mandible has a remarkable joint, quite unknown in land lizards, though a trace of it is found in the monitors, permitting much movement between the front and back parts, both laterally and vertically, though chiefly in the former direction. Furthermore, as in land snakes but not as in land lizards, the front ends of the two sides of the jaws were somewhat loosely attached to each other by ligaments. This looseness of the two sides of the jaws, not only in front but also behind, together with the joint in each, was of the greatest use in swallowing prey, as will be explained farther on.

As in most other aquatic reptiles, the neck was short and strong, the vertebrae being less in number than in most other lizards. The trunk was long and slender, more especially so in the surface-swimming kinds, with from twenty-two to thirty-four vertebrae. The tail was long, no longer than the tail of some land lizards, but more powerful, and broader and flatter. It was expanded or dilated more or less toward the free end, that is, with the beginning of a terminal caudal fin, such as the more specialized ichthyosaurs and crocodiles possessed. The vertebrae were procoelous, that is, concave in front and convex behind, like those of most modern lizards and all modern snakes and crocodiles, but quite unlike the biconcave vertebrae of all other aquatic reptiles. This kind of articulation of the backbones gave greater firmness and strength to the spinal column, but decreased the flexibility, and its possession by these animals was doubtless due to their descent from land lizards which had already acquired it. The loss of flexibility, however, was partly compensated by the loss of the additional articulating surfaces of the tail.

As in all other aquatic reptiles, it is in the limbs that the most striking characteristics of these water lizards or “sea-serpents” are found. The legs were so completely adapted to an aquatic mode of living that the animals must have been practically helpless upon land, able perhaps to move about in a serpentine way when accidentally stranded upon the beaches, but probably never seeking the land voluntarily. The front limbs, like those of all other swimming animals having a powerful propelling tail, were larger than the hind ones, though not very much so. The bones of the first two segments, that is, the arm, forearm, and thigh and leg bones, were all short and broad, resembling those of the ichthyosaurs more than those of any other reptiles, save perhaps the thalattosaurs, discussed below. The articular surfaces of all the limb bones, as in other aquatic animals, were restricted in extent, indicating limited motion between the joints, though doubtless having great flexibility. In the most specialized types, such as Tylosaurus, the wrist and ankle bones were almost wholly cartilaginous, just as they are in the water salamanders, and in whales and porpoises. This tendency of the ends of long bones, the wrists and ankles as well as other bones of the skeleton, to become more cartilaginous, or less well ossified, in animals purely aquatic in habit is a marked one. So much is this the case that paleontologists always suspect water habits in reptiles showing it, even though but few parts of the skeleton are known.

Increase in the number of bones of the digits is a more or less conspicuous characteristic of all mosasaurs. In those forms in which the wrists and ankle bones had become cartilaginous in great part, as many as eleven phalanges have been observed in the longest toes, though in other forms, those with more completely ossified wrists and ankles, only two or three additional bones have been developed in the longest fingers and toes by aquatic habits. The pliability and flexibility of the fingers and toes were certainly very great, but they could not possibly have been flexed or bent so as to grasp or seize anything; and of course all vestiges of claws had disappeared. Many specimens have been found with all the bones of the limbs, that is, the “paddle bones,” in the positions they occupied when the animals died. Figures of three such specimens, made from photographs or careful drawings by the writer, are shown herewith (Figs. 74-76). In several such specimens very clear impressions of the smooth membranes between the fingers have been observed, and in one specimen preserved in the collections of the University of Kansas the outline of the fleshy parts connecting the paddle with the body has been preserved.

It will be seen by comparison of the figures of the mosasaur paddles with those of the ichthyosaurs and plesiosaurs that there was a wide difference in their structure, though all have the characteristic shortening of the limb bones and increase in the numbers of the finger and toebones, that is hyperphalangy. It is probable that these differences mean a more powerful and varied use of the limbs in the mosasaurs. It is certain that the mosasaurs were much more predaceous and pugnacious in their habits than were any other truly aquatic backboned air-breathing animals of the past or present. They were the “land sharks” of the ancient seas, and probably the only ones among water reptiles that would be dangerous and offensive to man, were they all living today.

For a long time it was thought that the mosasaurs had no breast bone, and that, in consequence, the front part of the thorax was expansible. Under this assumption the mosasaurs would have been much more snake-like in habit than they really were. The loose construction of the jaws doubtless permitted the swallowing of prey of considerable size, and the inference was that they habitually preyed upon animals of large size. A snake will often swallow a frog of larger diameter than its own body, the flexible jaws and loosely connected ribs permitting it to pass to the abdominal cavity. But the unyielding ring formed by the anterior ribs connected with the breast bone in the mosasaurs, as in other lizards, conclusively proves that large animals could not have been swallowed whole by the mosasaurs. In several instances the fossilized stomach contents, composed chiefly or wholly of fishes, have been found between the ribs of mosasaurs, and in none were the fishes more than two or three feet in length, though the reptiles were from sixteen to twenty feet long. Possibly the largest mosasaurs, those thirty or thirty-five feet in length, might have captured and swallowed fishes six or seven feet long, but in all probability their usual prey was of smaller relative size.

The very loose construction of the pelvic bones, those to which the hind legs are articulated, is an evidence of more complete adaptation to water life than was or is the case with any other water air-breathers except the ichthyosaurs and cetaceans. The sacrum had entirely lost its function as a support to the pelvis and had disappeared, that is, the vertebrae composing it had become quite like the adjacent ones, by the loss of the ribs connecting them with the ilium. The small pelvis was suspended loosely in the walls of the abdomen, or at the most was feebly connected with a single vertebra by ligaments. It was entirely useless as a support for the legs. The mosasaurs could not possibly have raised their bodies from the ground while on land. It is well known that the land lizards and the crocodiles raise their bodies free from the ground while running or walking; none drags its body over the surface.

In several instances complete or nearly complete skeletons of mosasaurs have been discovered with the different bones nearly all in the positions and relations they had after the decomposition of the flesh, together with the carbonized remains of the skin and impressions of the investing scales and membranes. The nature of the body covering is therefore known with certainty from nearly all parts of the body. The body everywhere, save on the membrane between the fingers and toes, and perhaps on the top of the skull, was covered with small overlapping scales, very much like those of the monitors. These scales, however, were small and smooth in comparison with the size of the animals, those of a mosasaur twenty feet in length being almost precisely the size of those of a monitor six feet long. The top of the skull seems to have been covered with horny plates, as in most lizards. In one instance parallel dark bars, obliquely placed, and of narrow width, formed by carbonized pigment, were observed by the writer. As has been stated, in some instances fish bones and fish scales have been observed among the fossilized stomach contents, and it is quite certain that the food of these creatures must have been composed chiefly of fishes, though of course it is not improbable that other small vertebrates, birds, pterodactyls, the young of plesiosaurs, and possibly small mammals, may occasionally have formed a part of their diet. That the mosasaurs were very pugnacious in life is conclusively proved by the many mutilations of their bones that have been observed, mutilations received during life and partly or wholly healed at the time of death. Bones of all vertebrates are repaired after injury by the growth of more or less spongy osseous material about the injured part, forming a sort of natural splint. This material is more or less entirely removed by absorption when it is no longer required for the support of the broken ends. Many such injured bones of the mosasaurs have been found; sometimes the bones of the hands and feet have grown together, and not infrequently the vertebrae have been found united by these osseous splints; occasionally even the skull itself, especially the jaws, attest extensive ante-mortem injuries. In a single instance the writer has observed the loss of a part of the tail, where it probably had been bitten off. It may be mentioned, however, that the bones of the tail had no such “breaking points” in the mosasaurs as have those of many land lizards, whereby a part or all may be lost as a result of even a trivial injury, and then regrown. Such a condition in an organ relied upon entirely for propulsion would have been immediately fatal to the existence of the mosasaurs. The large jaws and teeth are in themselves sufficient evidence of the fiercely carnivorous propensities of the mosasaurs. The constant renewal of the sharply pointed teeth, thereby preventing deterioration by use or accident, preserved, even in the oldest animals, the effectiveness of the youthful structure.

We may now understand how the mosasaurs seized and swallowed their prey. Living constantly in the water, away from all firm objects, with small, short limbs quite incapable of holding struggling prey, and the body not sufficiently serpentine to hold it in its folds after the manner of snakes, the mosasaurs would have found it difficult or impossible to swallow fishes of even moderate size, were their jaws of the same construction as are those of the land lizards. If they preyed upon small animals only, or if they tore their prey to pieces after the manner of the alligators, there would have been no especial difficulty in deglutition. But it is certain that the animals which the mosasaurs devoured were not always small, and they must have been swallowed whole, since their teeth were not adapted, like those of the alligators and true crocodiles, for the rending of bodies. One who has watched a snake swallow a frog or another snake will appreciate the difficulties against which the mosasaurs contended in swallowing fishes a fifth or a sixth of the length of their own bodies. The ordinary snake, no matter where or how it seizes its prey, invariably swallows it head first. Its mandibles are even more loosely united in front than were those of the mosasaurs, and while there is no joint in the snake’s mandibles such as there is in the mosasaurs’, the loose union of the various bones of the mandibles serves the same purpose. The frog or lizard, while firmly held by the slender teeth, is slowly moved sideways by the alternate lateral action of the jaws till the head is reached. Many non-poisonous snakes, if they find it impracticable or impossible to reverse the position of their prey in this way, wrap the folds of their body about it, holding it firmly while they release their mouth-hold and seize it by the head. An amusing instance of these habits came under the observation of the writer not long ago, in Texas. A large “blue racer” (Bascanion), six feet four inches in length, caught an unusually large bullfrog by one hind leg, but in almost less time than it takes to relate, the head of the frog had entered the snake’s gullet and the mouth was closed over it, notwithstanding the vigorous muscular and vocal protests on the part of the frog. Wishing to secure the skull of the snake for his collection, the writer seized an ax standing conveniently by and cut the snake cleanly in two. The peristaltic action of the deglutitional muscles carried the frog slowly on about two feet farther to the ax-made orifice, from which it emerged, and, after a few croakings against such unkind usage, calmly hopped off into the near-by pool of water! Many poisonous snakes release their prey after killing it; other snakes may force their prey down the throat by pressing it against the ground.

Even small fishes could not possibly have been swallowed by the mosasaurs in any other way than head first, since the backwardly projecting, and often long, spines would have rendered any other procedure impossible. Even after the head had entered the gullet, deglutition could have been effectively completed only by the aid of some mechanism whereby the fish could have been pulled or pushed back into the constricting fauces. The strong teeth of the upper jaws and palate held firmly the struggling prey, while the loosely united jaws, bending laterally at the joint back of the middle, either alternately, or more probably in unison, steadily forced it far enough back to be seized by the muscles of the fauces.

The shape of the mosasaurs, though slender, does not suggest extraordinary speed in the water; doubtless most of the fishes that lived in the seas with them could swim faster than they. Their prey was captured, for the most part at least, by sudden and quick lateral movements, for which their powerful and flexible paddles admirably adapted them.

It is a rather remarkable fact that, among the thousands of specimens of mosasaurs which have been collected during the past forty years in both Europe and North America, there never has been found one of a very young animal. Of almost all other animals occurring abundantly as fossils some specimens are sure to be discovered of young and even embryonic individuals. It is quite certain that all such voracious monsters as were the mosasaurs did not die of old age. Some specimens, it is true, have been found that were evidently not full grown animals, but the observed differences in the size of the fossil bones are not great. All are of adult or nearly adult animals. If the mosasaurs were oviparous, as were the ichthyosaurs, and probably the plesiosaurs, and as are some living land lizards, the apparently entire absence of embryonic bones associated with often nearly complete skeletons of the mosasaurs is inexplicable; certainly some mosasaurs must have died a short time before the birth of their young. But embryos have never been discovered, though numerous skeletons inclosing fossilized stomach contents have been found. From this fact it would seem very probable that the mosasaurs were oviparous, as are most other lizards. But this, after all, may be a hasty inference.

No known reptiles lay their eggs in the water. Perhaps there is some reason why the eggs of reptiles and birds, so different from those of fishes and amphibians, cannot hatch in water; and there is no good reason for supposing that the mosasaurs were exceptions to this rule. Unless carefully hidden or protected by the parent, the eggs or very young of the mosasaurs would have been subject to many and grave dangers. Fish eggs are usually small and produced in great numbers, thousands often being extruded from a single female. Among so many there is a greater probability that at least two will hatch and survive to maturity, reproducing their kind. It is unreasonable to suppose that the lizards of the past were more prolific of eggs than are their relatives now living; nor is it possible that their eggs could have been as small as are those of most fishes. Modern lizards seldom lay more than twenty-five or thirty eggs at a time; even the turtles, with their greater vicissitudes, seldom produce more than one hundred. The eggs of the mosasaurs were certainly large and few in number, and the young animals must have begun breathing air immediately after escaping from the shells. If the mosasaurs were oviparous they must have laid their eggs upon the shores and beaches, as do the sea-turtles and the Crocodilia. Nor is it at all probable that the female mosasaurs gave even that protection to their eggs or young that the crocodiles and turtles give. The young mosasaurs, perhaps reaching a foot in length, must have been left entirely to their own devices and their own fate at the very earliest stages of their independent careers.

The waters in which the mosasaurs abounded swarmed with many kinds of predaceous fishes, to say nothing of the hordes of their own kinds, all carnivorous in the highest degree, to all of which the tender saurians must have been choice food. Possibly the shallow waters of the bays and estuaries may have afforded protection to the newly hatched reptiles. It would seem probable that the female mosasaurs went up the rivers for a shorter or longer distance to lay their eggs or give birth to their young, and that the young reptiles remained in such relatively protected places until of a sufficient size to cope with the fierce enemies of the open seas. We know practically nothing of the inhabitants of the lakes and rivers during all the time in which the mosasaurs existed; and this perhaps is the real reason why we have never yet found a specimen of a young mosasaur.