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Wealden occur, proves that they were floated down the streams and rivers, with rafts of trees and other spoils of the land, till, arrested in their course, they sank down and became buried in the fluviatile sediments then in progress.

The state of the first discovered specimen of the Hylaosaurus is in this point of view highly instructive: many of the bones are crushed and splintered, yet the fractured portions remain near each other; the vertebræ are more or less displaced, yet they maintain relation to the positions they originally occupied; the bones of the fore-legs have been torn from their sockets, and this must have taken place before the specimen was imbedded in the mud and sand, for the glenoid cavities were filled with stone: these facts prove that the carcass of the original must have undergone mutilation before the bones were reduced to a skeleton; and that the dislocated and broken parts were held together by the muscles and integuments; in this state the trunk was borne down the stream, and at length sank into the mud of the delta, and formed a nucleus around which the stems and leaves of cycadeous plants and ferns were accumulated, and river shells became intermingled in the general mass.

The phenomena here contemplated cannot, I conceive, be satisfactorily explained upon any other supposition than that which implies a long transport, by the agency of streams and currents the carcasses of the colossal reptiles must have been exposed to such an action for a considerable time, and the source of the mighty river which flowed through the Country of the Iguanodon, must, therefore, like that of the Mississippi, have been far distant from the delta which in the course of innumerable ages accumulated at its mouth.'

1 See "Wonders of Geology," p. 444, and pp. 483–490.

CHAPTER III.

PART VII.

THE PLESIOSAURI.

ENALIOSAURIANS-STRUCTURE OF THE PLESIOSAURUS-DISCOVERY OF THE PLESIOSAURUS DOLICHODEIRUS BY MR. CONY BEARE-HABITS OF THE PLESIOSAURI PLESIOSAURUS HAWKINSII PLESIOSAURUS ARCUATUS PLESIOSAURUS MACROCEPHALUS-PLESIOSAURUS RUGOSUS-PLESIOSAURI OF THE WEALDEN.

ENALIOSAURIANS.—As in the tertiary and modern epochs, the predominant terrestrial vertebrata, the mammalia, were represented by aquatic forms of gigantic size which inhabited the sea, so also during the "Age of Reptiles," types of the prevailing class of land quadrupeds swarmed in the ocean, and equalled in numbers, and almost in magnitude, the cetaceans of the present day. These reptilian denizens of the seas of the secondary ages, constitute the order Enaliosauria, (i.e. marine saurians,) which comprises two principal groups; namely, the Ichthyosauri, or fish-like lizards; and the Plesiosauri, which, as the name implies, are more nearly allied to the saurians than the animals of the former division; both were air-breathing, cold-blooded, carnivorous, vertebrate animals, with two pairs of natatory extremities. The Plesiosauri first claim our attention, because the specimens of this genus are deposited in the cases of the Room at present under survey.'

The perfect skeletons which will now engage the visitor's attention, present a remarkable contrast with the isolated and

1 Dr. Buckland's "Bridgewater Treatise" contains an admirable exposition of their habits and organization; and "Brit. Assoc. Rep." 1839, an elaborate osteological investigation of both genera.

LIGN. 73. PLESIOSAURUS HAWKINSII; FROM THE LIAS STRATA, SOMERSETSHIRE. (nat. size.)

fractured bones and waterworn relics of the terrestrial reptiles whose remains formed the subject of the previous section of this chapter. The beautiful state of preservation of many of the Plesiosauri,—the entire skeleton, from the point of the muzzle to the extremity of the tail, lying in relief, as if it had sunk down quietly on the soft clay, and become petrified on the spot,-manifests how different were the conditions in which the strata of the Lias and the Wealden were deposited; while the exquisite manner in which the investing stone has been removed, attests the consummate skill and indefatigable zeal of the gentleman by whom these superb fossils were developed.'

I shall reserve for the next chapter an account of the circumstances under which the matchless series of Enaliosaurian remains in our National Museum were obtained; and restrict my remarks to the description of the individual specimens in the Wall-cases D, E, and F, of this Room; those in Case D are arranged as under :

WALL-CASE D. [4.]

Original specimen of Plesiosaurus dolichodeirus.
(Rev. W. D. Conybeare, "Geol. Trans." vol. i. Pl. XVIII.

Top.

Plesiosaurus Hawkinsii.-Pl. XXIV. of Mr. Hawkins's work.

Bones of the

Plesiosaurus dolichodeirus:

Plesiosaurus the under surface of lower jaw exposed,
arcuatus. and two paddles on the right side.

Bones of the
Plesiosaurus
arcuatus.

PLESIOSAURUS.-The animals of this genus present in their osteological structure a remarkable deviation from all known recent and fossil reptiles; uniting the characters of the head of a lizard, with the teeth of a crocodile, to a neck of inordinate length, and with such modifications of the ribs, the pectoral and pelvic arches, and the paddles, as to justify the graphic simile of an eloquent Professor, that the Plesiosaurus might be compared to a serpent threaded through the shell of a turtle.

The character which immediately strikes the observer, is

1 Thomas Hawkins, Esq. The splendid volume of this gentleman on these marine saurians cannot fail to delight the reader by its graphic descriptions, and beautiful illustrations. See APPENDIX K, Mr. Hawkins's Collection of Enaliosauria.

the extraordinary length of the neck, and the relative smallness of the head. The neck, which in most animals is formed of but five vertebræ, and in the extremest recent example, the Swan, does not exceed twenty-four, is in certain species of Plesiosaurus composed of from twenty to forty vertebræ, and is four times as long as the head, and equal in longitudinal extent to the body and tail; while the length of the head (in P. dolichodeirus) is less than one-thirteenth of the entire skeleton; the tail is very short.

The skull resembles that of the Crocodiles in its general form, but is proportionately smaller; the breathing apertures are situated anterior to the orbits, on the highest part of the head. The orbit is relatively large, and furnished with a zone of bony plates, as in the sclerotica of certain lizards and birds.1 The lower jaw has the usual structure of the saurians; but the dentary bone is greatly expanded anteriorly, and united in front.

The teeth are implanted in separate sockets, and there are from thirty to forty on each side the jaws. They are conical, slender, long, pointed, slightly recurved, and longitudinally grooved from the base upwards, and have a long round fang. The pulp-cavity is long and simple, surrounded by a body of firm dentine, covered on the crown with a layer of enamel, and at the base with cement. The dentition in the Plesiosauri differs from that of the Crocodiles, in the successional teeth emerging through distinct apertures on the inner side of the sockets of their predecessors, and not through the pulp-cavity. The vertebræ are relatively longer than in the Ichthyosaurus, and their articular faces are either flat, or slightly excavated towards the periphery, with a gentle convexity in the centre.

2

The Pectoral arch is remarkable for the greatly elongated and broad Coracoid bones. The Ribs, which are very numerous, and extend throughout a great portion of the vertebral column, are connected, anteriorly, by slender bones; the Ichthyosaurus has a similar structure. As these connecting parts are so constructed as to admit of a certain degree of gliding motion upon each other, it is inferred that consi

1 See Dr. Buckland's "Bridgewater Essay," Pl. X.
2 See Professor Owen's "Odontography,” p. 282.

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