Essay, by Dr. Buckland,-and the elaborate and highly philosophical review of the subject, and the determination of all the then known species, by Professor Owen, in 1839, in the Reports on British Fossil Reptiles," undertaken and published under the auspices of the British Association of Science, -we are indebted for the present advanced state of this department of British Paleontology. The number of species of both genera is now considerable, and many specimens, both of Ichthyosauri and Plesiosauri, have been discovered in various localities on the Continent.

THE LIAS FORMATION.-Although remains of Enaliosaurians occur in all the formations from the Muschelkalk below the Lias, to the Chalk inclusive, it is in the liassic deposits that the greatest number, and the most perfect examples, have been discovered; and as the specimens which now engage our attention were collected from those strata by Mr. Hawkins, I shall offer a few remarks on the geological characters of the localities whence these splendid fossils were obtained. The Lias, situated between the Triassic, or New Red Formation, and the Oolite, consists of a series of argillaceous limestones, marls, and clays, which may be regarded as forming the base of the latter formation, for there are scarcely sufficient grounds for their separation; the Upper Lias, and the Inferor Oolite which lies upon it, having many fossils in common, and in some localities passing into each other. The total thickness of the Lias varies from 500 to 1000 feet: the strata have a very uniform lithological character, and contain many peculiar organic remains.

The Lias appears beneath the Oolite, through the southeast of Somersetshire, and extends into Dorsetshire, forming a range of cliffs, above four miles in length, along the seashore at Lyme Regis, where it may be traced on the coast till it gradually sinks beneath the Inferior Oolite. Lyme Regis in Dorsetshire, and Watchet, Street, and Westbury, in Somersetshire, are the localities that have afforded the most instructive specimens.

The subdivisions of the Lias are characterised by the abundance of particular groups of fossils. Some of the strata

1 See "Wonders of Geology," p. 521; and Sir C. Lyell's "Elements of Geology," p. 273.

ROOM IV. STATE OF PRESERVATION OF ENALIOSAURIANS.

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contain a greater number, and more perfect skeletons of saurians, than others. The uppermost beds consist of Alum shale, with a profusion of ammonites and crinoidal remains. The next subdivision comprises strata of marlstone and blue marl, in which bones of Enaliosaurians are but rarely met with ; but ammonites, belemnites, and other cephalopoda, and the usual marine shells and zoophytes of the Liassic formation are abundant.

The next group, the limestones, is the grand depositary of the reptilian remains," the inestimable treasury of the most splendid epoch in the physical records of our planet." Some of the thin intermediate layers of stone are, however, literally a mass of pentacrinites, and others are wholly made up of ammonites; the organic remains being more or less mineralized by pyrites.2

The most beautiful and perfect examples of Plesiosauri and Ichthyosauri collected by Mr. Hawkins, were extracted from these strata. The total thickness of the limestones, and alternating layers of marl, at Kingston, near Street, is about twenty feet; at Lyme Regis (forty miles from Street), the section east of Church Cliff, is thirty feet thick.

A bituminous marl, of a black colour, the last deposit in the series, contains similar remains with the limestones; and in addition, some fossil terrestrial vegetables not observed in the other strata.3

At the base of the Lias, and separating the lowermost shale from the uppermost Triassic bed beneath, there is a layer of coarse detritus, a few inches thick, commonly known as the Bone-bed, composed of mud and sand, and the debris of fishes and reptiles.*

STATE OF PRESERVATION OF ENALIOSAURIANS.-The remarkably perfect state of the skeletons of the Plesiosauri has already been pointed out; many of those of the Ichthyosauri are equally entire. In several of the specimens in the Museum, the bones are seen in all their integrity, as in recent anatomi

1 Mr. Hawkins's Memoir, p. 5.

2 Specimen of Pentacrinites from these strata are deposited in Wallcase G, Room II. see ante, p. 74.

3 Mr. Hawkins's Memoir, p. 7.

4 See "Wonders of Geology," p. 529.

BB

cal preparations. Reflecting on the circumstance that the Enaliosaurians lived in an ocean which swarmed with predaceous fishes and other animals, and that both genera were carnivorous, and doubtless preyed on the young and the feeble of their own races, it seems difficult to account for the great number of entire carcasses that must have been buried in the mud at the bottom of the sea, and left unmolested on the spot where they died and became imbedded.

In explanation of this fact, Dr. Buckland has suggested the probability that these creatures experienced a sudden death, from a diffusion of mephitic vapours through the water, by a submarine volcanic eruption; such a phenomenon, it is inferred, would account for the destruction of shoals of those reptiles that were within the reach of the gaseous influence, and at the same time prevent the approach of any predaceous animals, till the carcasses were enveloped in the mud, and placed beyond the reach of assailants.

In connexion with the facts referred to, it is worthy of remark that the Ichthyosauri are, for the most part, found lying on the side, while the Plesiosauri are extended on their backs, with the abdominal region uppermost, as in the splendid fossil figured in Lign. 73. It has been very ingeniously conjectured by Mr. Samuel Stuchbury,' that this difference of position is referable to the form and structure of the animals of the two genera. In the case of the Plesiosauri it is assumed, that after death, the gases evolved by putrefaction from the abdominal viscera, were retained by the tough dermal integuments and the sterno-costal arcs, and the body was thus suspended with the belly uppermost, till it became water-logged, and buried in the silt. Whereas the fish-like form of the Ichthyosauri,—the vertical diameter of the body being greater than the lateral,-is presumed to account for the lateral position of the fossil skeletons; while the frequently dislocated state of the bones is supposed to have resulted from the strong integuments of the body having remained entire, until the internal parts were decom

1 "Description of a new species of Plesiosaurus (P. megacephalus), in the Museum of the Bristol Institution." By Samuel Stuchbury, Esq. F.G.S. "Geolog. Journal," vol. ii. 1846, p. 411.

posed and the bones disunited, but kept from dispersion, as in a sack, till the whole mass was imbedded.

STRUCTURE OF THE ICHTHYOSAURUS.-The Ichthyosaurus, though agreeing in its general characters with the Plesiosaurus, presents a much nearer approach to the fishes than that genus of Enaliosaurians. From the shortness of the neck, and great breadth of the base of the cranium, its general outline must have closely resembled that of a large Porpoise, or Grampus, with enormous eyes, two pairs of fins or paddles, a long tail, and, probably, a large integumentary caudal fin.

The ver

The internal structure corresponds with the outer form in its close approximation to fishes or cretaceans. tebræ have their articular surfaces so deeply cupped, that it is inferred "they were originally connected together by an elastic capsule, filled with a fluid, as in the vertebral joints of fishes, and the perennibranchiate, or most fish-like, of Reptiles." 1

The muzzle of the Ichthyosaurus is long and pointed; the lower jaw is formed of two branches, united anteriorly through nearly half their length; each branch is composed of six bones, as in the Crocodiles and Lizards, but differently arranged than in those reptiles.

The teeth are very numerous, amounting to nearly two hundred in some species, and are placed in a single row on each side the jaws, being implanted in a deep continuous groove without sockets. These teeth are of a pointed conical form, longitudinally striated, with an expanded base. The new teeth are developed at the inner side of the base of the old ones, and grow up and displace them. The tooth consists of a pulp-cavity, surrounded by a body of dentine, which is invested at the base by a thick layer of cement, and at the crown by a coat of enamel; the pulp-cavity, in fully-formed teeth, is more or less occupied by coarse bone.2 The chief peculiarity of this structure consists in the inflection of the cement into vertical folds at the base of the tooth, by which the marginal portion of the basal dentine is divided into a corresponding number of processes, as in the

1 Professor Owen, on the Ichthyosaurus.

2 See Prof. Owen's "Odontography,” p. 275.

"Brit. Assoc. Rep." 1839,

tooth of the Lepidosteus, an existing genus of river fishes. (See "Medals of Creation," Pl. VI. fig. 9.).

The nasal apertures, or openings of the nostrils, are placed as in the Plesiosaurus, near the anterior angle of the orbit.

The orbits are very large, and the sclerotic coat, or capsule of the eye, has in front an annular series of bony plates, which often occur in their natural position, and are beautifully displayed in many of the specimens in the Cases before us. This mechanism is not possessed by fishes, but is analogous to that observable in the eyes of turtles, lizards, and many birds; as, for example, in the owl and eagle: it confers on the eye additional power of adaptation, and intensity of vision.

The vertebræ, of which there are upwards of one hundred and forty in the individuals of some species, are, as we have already remarked, very short in their antero-posterior diameter (i.e. from front to back), and deeply cupped on each articulating face. The annular part is not united to the body of the vertebra, as in quadrupeds, nor connected by suture, as in Crocodiles, but terminates on each side in a compressed oval base, which fits into corresponding sockets placed on the boundary line of the spinal depression on the body, and thus completes the neural canal. The first and second vertebræ are anchylosed together, and have additional sub-vertebral, wedge-shaped bones, which render this part of the column a fixed point of support.1

The pectoral arch presents a remarkable resemblance to that of the Ornithorhynchus, or Duck-billed Platypus of New Holland. The episternum is of a triradiate form; the clavicles are strong, elongated, and slightly curved; the scapulæ are stout and broad, and in the form of a parallelogram; the coracoids are of a hatchet-shape, with a strong, thick, glenoid extremity, and a thin mesial expansion, which articulates with the episternal bone.2

The pelvic arch consists of the same elements as in the Plesiosaurus, but the ischium and pubis are much smaller than in that genus. The ilium is a short, simple, strong, and compressed bone, slightly expanding as it descends, to com

1 This structure was first demonstrated by Sir Philip Egerton. See "Geol. Trans. Second Series," Vol. V. p. 187, Pl. XIV.

2 See Dr. Buckland's "Bridgewater Essay," Pl. XII.