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This hypothesis of Cuvier is but conjectural, respecting the power of the Plesiosaurus to change the colour of its skin; and to the unexperienced in comparative anatomy, it may seem equally conjectural, to deduce any other conclusions respecting such perishable organs as the lungs, from the discovery of peculiar contrivances, and unusual apparatus in the ribs; yet we argue on similar grounds, when from the form and capabilities of these fossil ribs, we infer that they were connected, as in the Chameleon, withvast and unusual powers of expansion and contraction in the lungs; and when, on finding the ribs and wood-work of a worn-out bellows, near the ruins of a blacksmith'sforge, we conclude that these more enduring parts of the frame of this instrument, have been connected with a pro-portionable expansion of leather.

The compound character of the ribs, probably also gave to the Plesiosaurus the same power of compressing air within its lungs, and in that state taking it to the bottom, which we have considered as resulting from the structure of the sterno -costal apparatus of the Ichthyosauri.

Extremities.*

As the Plesiosaurus breathed air, and was therefore obliged to rise often to the surface for inspiration, this necessity was met by an apparatus in the chest and pelvis, and in the bones of the arms and legs, enabling it to ascend and descend in the water after the manner of the Ichthyosauri and Catecea; accordingly the legs were converted into paddles, longer and more powerful than those of the Ichthyosaurus, thus compensating for the comparatively small assistance which it could have derived from its tail.f

* See PI. 16, 17, 19.

t The number of joints representing the phalanges of the ringers and toes exceeds that in the Lizards and Birds, and also in all Mammalia; excepting the Whales, some of which present a similar increase of num. ber to accommodate them to the corresponding office of a paddle. The mode of connexion between the joints was (like that in the Whales,) by synchon-. drosis. The phalanges of the Plesiosaurus present a link, between the still more numerous and angular joints of the paddle of the Ichthyosaurus, and the phalanges of land quadrupeds, which are more or less cylindrical; in these sea Lizards they were flattened, for the purpose of giving breadth to the extremities as organs of swimming. As its paddles give no indication of having carried even such imperfect claws, as those of the Turtles and Seals, the Plesiosaurus apparently could have made little or no progress in any other element than water.

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Comparing these extremities with those of other vertebrated animals, we trace are gular series of links and gradations, from the corresponding parts of the highest mammalia, to their least perfect form in the fins of fishes. In the fore paddle of the Plesiosaurus, we have all the essential parts of the fore-leg of a quadruped, and even of a human arm; first the spapula, next the humerus, then the radius and ulna, succeeded by the bones of the carpus and metacarpus, and these followed by five fingers, each composed of a continuous series of phalanges. (See PI. 16, 17, 19.) The hind paddle also offers precisely the same analogies to the leg and foot of the Mammalia; the pelvis and femur are succeeded by a tibia and fibula, which articulate with the bones of the tarsus and metatarsus, followed by the numerous phalanges of five long toes.

From the consideration of all its characters, Mr. Conybeare has drawn the following inferences with respect to the habits of the Plesiosaurus Dolichodeirus, "That it was aquatic is evident, from the form of its paddles; that it was marine is almost equally so, from the remains with which it is universally associated; that it may have occasionally visited the shore, the resemblance of its extremities to those of the Turtle may lead us to conjecture; its motion however must have been very awkward on land; its long neck must have impeded its progress through the water; presenting a striking contrast to the organization which so admirably fits the Ichthyosaurus to cut through the waves. May it not therefore be concluded (since, in addition to these circumstances, its respiration must have required frequent access of air,) that it swam upon, or near the surface; arching back its long neck like the swan, and occasionally darting it down at the fish which happened to float within its reach. It may perhaps have lurked in shoal water along the coast, concealed among the sea-weed, and raising its nostrils to a level with the surface, from a considerable depth, may have found a secure retreat from the assaults of dangerous enemies; while the length and flexU bility of its neck may have compensated for the want of strength in its jaws, and its incapacity for swift motion through the water, by the suddenness and agility of the attack which they enabled it to make on every animal fitted for its prey, which came within its reach."—Geol. Trans, N. s. vol. i. part ii. p. 388.

We began our account of the Plesiosaurus with quoting the high authority of Cuvier, for considering it as one of the most anomalous and monstrous productions of the ancient systems of creation; we have seen in proceeding through our examination of its details, that these apparent anomalies consist only in the diversified arrangement, and varied proportion, of parts fundamentally the same as those that occur in the most perfectly formed creatures of the present world.

Pursuing the analogies of construction, that connect the existing inhabitants of the earth with those extinct genera and species which preceded the creation of our race, we find an unbroken chain of affinities pervading the entire series of organized beings and connecting all past and present forms of animal existence by close and harmonious ties. Even our own bodies, and some of their most important organs, are brought into close and direct comparison with those of reptiles, which, at first sight, appear the most monstrous productions of creation; and in the very hand and fingers with which we write their history, we recognise the type of the paddles of the Ichthyosaurus and Plesiosaurus.

Extending a similar comparison through the four great classes of vertebral animals, we find in each species a varied adaptation of analogous parts, to the different circumstances and conditions in which it was intended to be placed. Ascending from the lower orders, we trace a gradual advancement in structure and office, till we arrive at those whose functions are the most exalted: thus, the fin of the fish becomes the paddle of the reptile Plesiosaurus and Ichthyosaurus; the same organ is converted into the wing of the Pterodactyle, the bird and bat; it becomes the fore-foot, or paw, in quadrupeds that move upon the land, and attains its highest consummation in the arm and hand of rational man.

1 will conclude these observations in the words and with the feelings of Mr. Conybeare, which must be in unison with those of all who had the pleasure to follow him through his masterly investigations of this curious subject, from which great part of our information respecting the genus Plesiosaurus has been derived:

"To the observer actually engaged in tracing the various links that bind together the chain of organized beings, and struck at every instant by the development of the most beautiful analogies, almost every detail of comparative anatomy, however minute, acquires an interest, and even a charm; since he is continually presented with fresh proof of the great general law, which Scarpa himself, one of its most able investigators, has so elegantly expressed: 'Usque adeo natura, una eadem semper atque multiplex, disparibus etiam formis effectus pares, admirabili quadam varietatum simplicitate conciliat.'"

SECTION VII.

MOSASAURUS, OR GREAT ANIMAL OF MAESTRICHT.

The Mosasaurus has been long known by the name of the great animal of Maestricht, occurring near that city, in the calcareous freestone which forms the most recent deposite of the cretaceous formation, and contains Ammonites, Belemnites, Hamites, and many other shells belonging to the chalk, mixed with numerous remains of marine animals that are peculiar to itself. A nearly perfect head of this animal was discovered in 1780, and is now in the Museum at Paris. This celebrated head during many years baffled all the skill of Naturalists; some considered it to be that of a Whale, others of a Crocodile; but its true place in the animal kingdom was first suggested by Adrian Camper, and at length Confirmed by Cuvier. By their investigations it is proved to have been a gigantic marine reptile, most nearly allied to the monitor.* The geological epoch at which the Mosasaurus first appeared, seems to have been the last of the long series, during which the oolitic and cretaceous groups were in process of formation. In these periods the inhabitants of our planet seem to have been principally marine, and some of the largest creatures were Saurians of gigantic stature, many of them living in the sea, and controlling the excessive increase of the then existing tribes of fishes.

From the lias upwards, to the commencement of the

* The Monitors form a genus of Lizards, frequenting marshes and the banks of rivers in hot climates; they have received this name from the prevailing, but absurd, notion that they give warning by a whistling noise, of the approach of Crocodiles and Caymans. One species, the Lacerta nilotica ( which devours the eggs of Crocodiles, has been sculptured on the monuments of ancient Egypt.

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