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chalk formation, the Ichthyosauri and Plesiosauri were the tyrants of the ocean; and just at the point of time when their existence terminated, during the deposition of the chalk, the new genus Mosasaurus appears to have been introduced, to supply for a while their place and office,* being itself destined in its turn to give place to the Cetacea of the tertiary periods. As no Saurians of the present world are inhabitants of the sea, and the most powerful living representatives of this order, viz. the Crocodiles, though living chiefly in water, have recourse to stratagem rather than speed, for the capture of their prey, it may not be unprofitable to examine the mechanical contrivances, by which a reptile, most nearly allied to the Monitor, was so constructed, as to possess the power of moving in the sea, with sufficient velocity to overtake and capture such large and powerful fishes, as from the enormous size of its teeth and jaws, we may conclude it was intended to devour.

The head and teeth, (PI. 20.) point out the near relations of this animal to the Monitors; and the proportions maintained throughout all the other parts of the skeleton, warrant the conclusion that this monstrous Monitor of the ancient deep was five and twenty feet in length, although the longest of its modern congeners does not exceed five feet. The head here represented measures four feet in length, that of the largest Monitor does not exceed five inches. The most skilful Anatomist would be at a loss to devise a series of modifications, by which a Monitor could be enlarged to the length and bulk of a Grampus,f and at the same time be fitted to move with strength and rapidity through the waters of the sea; yet in the fossil before us, we shall find the genuine characters of a Monitor maintained throughout the whole skeleton, with such deviations only as tended to fit the animal for its marine existence.

* Remains of the Mosasaurus have been discovered by Mr. Mantell in the upper chalk near Lewes, and by Dr. Morton in the green sand of Virginia.

t The Grampus is from 20 to 25 feet long, and very ferocious, feeding on seals and porpoises as well as on other fishes.

The Mosasaurus had scarcely any character in common with the Crocodile, but resembled the Iguanas, in having an apparatus of teeth fixed on the pterygoid bone, (PI. 20. k.) and placed in the roof of its mouth, as in many serpents and fishes, where they act as barbs to prevent the escape of their prey.*

The other parts of -the skeleton follow the character indicated by the head. The vertebrae are all concave in front, and convex behind; being fitted to each other by a ball and socket joint, admitting easy and universal flexion. From the centre of the back to the extremity of the tail, they are destitute of articular apophyses, which are essential to support the back of animals that move on land: in this respect, they agree with the vertebrae of Dolphins, and were calculated to facilitate the power of swimming; the vertebrae of the neck allowed to that part also more flexibility than in the Crocodiles.

The tail was flattened on each side, but high and deep in the vertical direction, like the tail of a Crocodile; forming a straight oar of immense strength to propel the body by horizontal movements, analogous to those of skulling. Although the number of caudal vertebras was nearly the same as in the Monitor, the proportionate length of the tail was much diminished by the comparative shortness of the body of each vertebra; the effect of this variation being to give strength to a shorter tail as an organ for swimming; and a rapidity of movement, which would have been unattainable by the long and slender tail of the Monitor, which assists that animal in climbing. There is a farther provision to give strength to the tail, by the chevron bones being soldered firmly to the body of each vertebra, as in fishes.

* The teeth have no true roots and are not hollow, as in the Crocodiles, but when full grown, arc entirely solid, and united to the sockets by a broad and firm base of bone, formed from the ossification of the pulpy matter which had secreted the tooth, and still farther attached to the jaw by the ossification of the capsule that had furnished the enamel. This indurated capsule, passed like a circular buttress around its base, tending to make the tooth an instrument of prodigious strength. The young looth first appeared in a separate cell in the bone of the jaw, (11. 20, h.) and moved irregularly across its substance, until it pressed against the base of the old tooth; causing it gradually to become detached, together with its base by a kind of nrcrosU> and to fall off like the horns of a Deer. The teeth, in the roof of the mouth, are also constructed on the same principle with those in the jaw, and renewed in like manner. VOL. I. 15

The total number of vertebrae was one hundred and thirty-three, nearly the same as in the Monitors, and more than double the number of those in the Crocodiles. The ribs had a single head, and were round, as in the family of Lizards. Of the extremities, sufficient fragments have been found to prove that the Mosasaurus, instead of legs, had four large paddles, resembling those of the Plesiosaurus and the Whale: one great use of these was probably to assist in raising the animal to the surface, in order to breathe, as it apparently had not the horizontal tail, by means of which the Cetacea ascend for this purpose. All these characters unite to show that the Mosasaurus was adapted to live entirely in the water, and that although it was of such vast proportions compared with the living genera of these families, it formed a link intermediate between the Monitors and the Iguanas. However strange it may appear to find its dimensions so much exceeding those of any existing Lizards, or to find marine genera in the order of Saurians, in which there exists at this time no species capable of living in the sea; it is scarcely less strange than the analogous deviations in the Megalosaurus and Iguanodon, which afford examples of still greater expansion of the type of the Monitor and Iguana, into colossal forms adapted to move upon the land. Throughout all these variations of proportion, we trace the persistence of the same laws, which regulate the formation of living genera, and from the combinations of perfect mechanism that have, in all times, resulted from their operation, we infer the perfection of the wisdom by which all this mechanism was designed, and the immensity of the power by which it has ever been upheld.

Cuvier asserts of the Mosasaurus that before he had seen a single vertebra, or a bone of any of its extremities, he was enabled to announce the character of the entire skeleton, from the examination of the jaws and teeth alone, and even from a single tooth. The power of doing this results from those magnificent laws of co-existence, which form the basis of the science of comparative anatomy, and which give the highest interest to its discoveries.

SECTION VIII.

PTERODACTYLE.*

Among the most remarkable disclosures made by the researches of Geology, we may rank the flying reptiles, which have been ranged by Cuvier under the genus Pterodactyle; a genus presenting more singular combinations of form, than we find in any other creatures yet discovered amid the ruins of the ancient earth .f

The structure of these animals is so exceedingly anomalous, that the first discovered Pterodactyle (PI . 21) was considered by one naturalist to be a bird, by another as a species of bat, and by a third as a flying reptile.

This extraordinary discordance of opinion respecting a creature whose skeleton was almost entire, arose from the presence of characters apparently belonging to each of the three classes to which it was referred. The form of its head, and length of neck, resembling that of birds, its wings approaching to the proportion and form of those of bats, and the body and tail approximating to those of ordinary Mammalia. These characters, connected with a small skull, as is usual among reptiles, and a beak furnished with not less than sixty pointed teeth, presented a combination of apparent anomalies which it was reserved for the genius of Cuvier to reconcile. In his hands, this apparently monstrous production of the ancient world, has been converted into one of the most beautiful examples yet afforded by comparative anatomy, of the harmony that pervades all nature, in the adaptation of the same parts of the animal frame, to infinitely varied conditions of existence.

• See PI. 1, Figs. 42, 43, and Plates 21,22.

j- Pterodactyles have hitheilo been found chiefly in the quarries of lithographic limestone of the jura formation at Aichstadt and Solenhofen; a stone abounding in marine remains, and also containing Libellulae, and other insects. They have also been discovered in the lias of Lyme Regis, and in oolitic slate of Stonesfield.

In the case of the Pterodactyle we have an extinct genus of the Order Saurians, in the class of Reptiles, (a class that now moves only on the land or in the water,) adapted by a peculiarity of structure to fly in the air. It will be interesting to see how the anterior extremity, which in the fore-leg of the modern Lizard and Crocodiles is an organ of locomotion on land, became converted into a membraniferous wing; and how far the other parts of the body are modified so as to fit the entire animal machine for the functions of flight. The details of this inquiry will afford such striking examples of numerical agreement in the component bones of every limb, with those in the corresponding limbs of living Lizards, and are at the same time so illustrative of contrivances for the adjustment of the same organ to effect different ends, that I shall select for examination a few points, from the long and beautiful analysis which Cuvier has given of the structure of this animal.

The Pterodactyles are ranked by Cuvier among the most extraordinary of all the extinct animals that have come under his consideration; and such as, if we saw them restored to life, would appear most strange, and most

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