FOR many years it has been known that the stomach cavities of these animals frequently contain the remains of little skeletons. At first it was naturally assumed that they were the remains of food, but a more complete and detailed examination of many specimens has proved that the species to which the little remains belong is always the same species as the ‘devouring ‘animal. Such reptiles can hardly be credited with preferring only their own very limited species as food, and the inevitable conclusion is that these little forms are not the remains of food but are the skeletons of embryos or unborn young. This conclusion has been greatly strengthened by recent discoveries and may, in fact, be taken as proved.

As the ichthyosaurs were thus not obliged to return to land to lay eggs, they enjoyed a much greater measure of freedom than the plesiosaurs, and are amongst the most successfid of all the creations for water-living.

Towards the close of the Cretaceous* period, there came into being a group of marine reptiles which had a short geological history but were world-wide in distribution. Although confined to the period when the chalk was being deposited, remains have been found in Kent, Belgium, Holland, New Zealand, Nigeria, Canada, and in the United States. This group is known as the Mosasaurs, or ‘Mcuse-reptiles,’ since the first specimens were found at Maastricht in the valley of the Meuse.

In appearance they were like lengthened ichthyosaurs or porpoises, with a reduced skeletal structure adapted to their marine habitat. Their somewhat fragile skull bore strong, recurved teeth not only on the margins of the jaws but on some of the palatal bones as well. Further, the connections between the jaw bones were loose, like those of snakes, so that bulky prey could be swallowed. Propulsion was effected by means of flattened lizard-like limbs used as paddles, but some forms may also have had the assistance of a long but narrow tail fin. The skin was probably smooth, but may have had a thin covering of scales.

Some mosasaurs grew to fifty feet or more in length. Their thin bodies, capable of high speed in the water, and their cruel teeth made them the most rapacious animals that have ever lived in the sea. They were, indeed, the sea-serpents par excellence, but have been long extinct for reasons we cannot guess.

A STRANGE AND SUDDEN DISAPPEARANCE ALL these marine reptiles which lived so long and with such conspicuous success suddenly fade out of the picture at the close of the Cretaceous period. No new forms of any importance had been introduced to alter the tenor of their lives. For the whole of their time, they had suffered the competition of the fish without the least discomfort. Suddenly and for no explicable reason, except that they had had their day, they vanished from the world of living things merely to become a portion of geological history.

During their time, the crocodiles had also been developing and becoming comparatively abundant. The earlier crocodiles were more adapted for a marine life than the later and living forms. One important difference is that they lacked the secondary bony palate which enables the present-day crocodiles to drown their prey and yet breathe comfortably themselves. Unless, therefore, these earlier types were provided with some compensating fleshy structure of which all trace is lost in the fossils, their mode of life and feeding habits must have been different from those of their representatives to-day.

The later crocodiles are so closely akin to living forms that no mention need be made of them here, beyond pointing out that certain Indian forms of the Pliocene age grew to the enormous length of fifty feet.

While these swimming reptiles sported in the seas, the shadows of others of their relatives in another element no doubt fell upon them; for at this time, reptiles, like many other vertebrate groups, attempted the conquest of the air. These flying reptiles are generally known as Pterodactyls (‘wing fingers ‘), because the thin skin that served as a wing was attached to the greatly lengthened fifth finger.

The early pterodactyls were small, about the size of sparrows; some had short tails, others had long ones. They are especially well known from the Lithographic Stone of Bavaria, where slabs of rock often reproduce with great clarity the delicate impressions of complete individuals.

Pterodactylus itself was a small animal like a thrush, with a long, bird-like skull set at right angles to the neck. The slender jaws were toothed. The arms were strongly developed compared with the legs, and the latter could not have been of much value for movement. The breast bone was keeled for the insertion of muscles, as in birds.

The wing-membrane consisted essentially of three parts : a little flap of skin between the forearm and the neck, a large area of this skin between the disproportionately strengthened ‘little ‘finger and the hind-leg, and a small area between the hind-legs. In some forms at least there are indications of birdlike habits, and a certain amount of flying in the strict sense may have been accomplished. Others, it seems, only glided on wind currents.

A GIANT REPTILE THAT COULD NOT USE ITS FEET LIVING at the same time as Pterodactylus was another genus vof very similar nature known as Rhamphorhynchus. It had, however, a long tail with a sort of little fin at the end. In its mouth were many slender teeth, but the front of the mouth was toothless and beaked. Impressions of the wing-membrane have been found which show that it was smooth, and the body appears to have been quite smooth and un-armoured also.

The later pterodactyls, those of the Cretaceous period, were much larger than their predecessors. The most notable form was Ptera?wdon, whose remains have been found in the Chalk of Kansas, U.S.A. This animal had a large skull with long but toothless jaws. The fore-limbs were enormously developed and the span from wing-tip to wing-tip was no less than eighteen feet. The long beak in the head was counterbalanced by a strong bony process on the back of the skull, and to this process it is thought the muscles actuating the wing-fingers were attached. Apart from the great fifth finger, the hand had three little fingers with claws. The hind-legs were absurdly small when contrasted with the front limbs, and must have been too weak to support the creature. Certainly these pterodactyls could not have walked about like modern flying birds.

The bones are remarkably light in weight and construction, but when we try to arrange the mental picture of the life of these larger flying reptiles, we come up against some pretty problems. How and where did they live, and how did they catch their food? We have seen that most of them seem to have been poor fliers and to have been mainly wind-gliders : we see, too, that the hind-legs could hardly have served for walking; so that obviously the life of pterodactyls was not at all like that of most birds.

We can only assume that forms like Pteranodon launched themselves off from a cliff in a favourable wind and glided

over the water where they caught fish. What happened when they landed on the water, as they must occasionally have done, and were becalmed, we do not know. The fragile wing-tip, too, would easily be broken in contact with the choppy seas. But at any rate that is how they must have lived; floating on the air streams, perching on the rocks, finding food in the seas or amongst their flying fellows, the insects. Like so many of the other reptilian groups, they disappeared without descendants or survivors at the end of the Cretaceous period.