Such being the structure and the inferences justly deduced from it, we shall next see how accident may be improved by a good observer.

The very frequent occurrence of the fracture of the tail about one-fourth of the way from its distal extremity, had led Professor Owen to suspect that the accident was connected with the presence of a tegumentary caudal fin; and the laterally compressed form of the terminal vertebræ since ascertained by Sir Philip Grey Egerton, afforded additional demonstration both of its existence and direction. The only evidence in fact, as Professor Owen observes, which the skeleton of the cetaceous mammal gives of the powerful horizontal caudal-fin which characterizes the recent animal, is the depressed or horizontally flattened form of the terminal vertebræ. He therefore infers, from the corresponding vertebræ of the Icthyosaur being flattened in the vertical direction, or from side to side, that it possessed a caudal tegumentary fin expanded in the vertical direction: and he recommends a narrow examination of the lias matrix, in which the tail may have been imbedded for the traces of carbonaceous discoloration, or of an impression of this fin, from which some idea might be formed of its shape and size.

The occurrence of such a desired impression is not so improbable as those unacquainted with the subject may suppose. Dr. Buckland described the tegument of the abdomen, and Professor Owen that of the fin, from specimens found at Barrowon-Soar.

"Thus," says Professor Owen, "in the construction of the principal natatory organ of the Ichthyosaurus we may trace, as in other parts of its structure, a combination of mammalian, saurian, and ichthyic peculiarities. In its great length and its gradual diminution we perceive the saurian character; its tegumentary nature, unsupported by osseous rays, bespeaks its affinity to the cetaceans; while its vertical position brings it close to the peculiar condition of the natatory organ in the fish.

"But," continues the professor, "it may be argued, the horizontality of the caudal fin of the Cetacea is essentially connected with their exigencies as breathers of the atmospheric air: without this means of displacing a mass of water in a vertical direction, the head of the whale could not have been brought with the required rapidity and facility to the surface to inspire: and as the ichthyosaurus was also an air-breather, a like position of the caudal fin might be considered to be equally essential to its existence in the water."

To this objection, the professor replies that the Ichthyosaurus

not being warm-blooded, would not need to bring its head to the surface so frequently, or perhaps so rapidly, as the cetacean; and, moreover, a compensation for the absence of a horizontal terminal fin is provided in the presence of the two posterior paddles, which are wholly deficient in the Cetacea.

The professor's conception of the appearance of this bygone form, "in his habit as he lived," is, that the animal must have presented the general external figure of a huge predatory abdominal fish, with a longer tail and smaller caudal fin than usual; scaleless, moreover, and covered with a smooth or finely-wrinkled skin analogous to that of the Cetacea. But a closer inspection of the enduring parts of these singular inhabitants of the ancient deep, shows, he justly observes, that under their fishlike exterior was concealed an organization which, in the main is a modification of the saurian type.

A word or two now as to the enormous and curiously constructed eye of the Ichthyosaur, which must have possessed great visual powers, always on the watch to minister to its predaceous habits and to preserve it from the attacks of the larger individuals of its own kind; for that the Ichthyosaurs preyed on each other as well as on the Plesiosaurs and fishes, is as clearly proved as that the pike will dine on the pickerel.

This eye was both a microscope and a telescope, modified in its action by an apparatus similar to that which exists among the feathered tribes, and is most highly developed in the birds of prey. Like the eagles and the owls, the Ichthyosaurus was furnished with a bony sclerotic ring, or circle of osseous plates, arranged around the aperture where once the pupil glittered, and which with its attendant muscles altered the convexity of the cornea, so as to adjust the scope according to the necessities of the animal. Was a near object to be examined? this machinery by the retraction of the plates protruded the eye, which thus became microscopic. Was distant vision required?—the plates resumed their ordinary position and a telescopic range was secured. Here, in short, was an instrument to light the Ichthyosaur by day or by night, near the surface and in the deep, armed against external injury in the first-named locality, and against the pressure to which it must have been frequently subjected in the second.

As far as the known specimens can lead us to conclusions with regard to size, some of the species of the Ichthyosaurus, when full grown, must have exceeded thirty feet in length. The great relative proportion of the eye may be imagined from Dr. Buckland's declaration that it was sometimes larger than a man's head.

Ten species of these extinct Sea-dragons are enumerated by

Professor Owen, four described by the Rev. W. Conybeare, one by Mr. Koenig, and five by himself.

PLESIOSAURUS.

With much of similarity in general form to the Ichthyosaur, the ancient Plesiosaur, its contemporary, presents considerable differences even in external appearance. The first and most striking is the excessive length of the neck and the comparative smalless of the head. Cuvier says of this inhabitant of the ancient world of waters, that it is the most heteroclite of forms and one which seems best to deserve the name of monster.

"To the head of the lizard," says Dr. Buckland, "it united the teeth of the crocodile; a neck of enormous length, resembling the body of a serpent; a trunk and tail having the proportions of an ordinary quadruped; the ribs of a chamelion; and the paddles of a whale."

To the Rev. W. Conybeare and Sir Henry de la Beche we are indebted for the notice of this most extraordinary addition to the ancient Fauna, the first specimens of which appear to have been discovered about twenty years ago.

To form the head of a Plesiosaur we must combine the characters observable in the Ichthyosaur, the crocodile, and the lizard; but to that of the last-named saurian, the head of the Plesiosaur bears the nearest approximation.

"It agrees," says Dr. Buckland, with the Ichthyosaurus in the smallness of its nostrils, and also in their position near the anterior angle of the eye; it resembles the crocodile in having the teeth lodged in distinct alveoli; but differs from both in the form and shortness of its head, many characters of which approach closely to the Iguana."

The teeth are comparatively slender and sharp-pointed.

The swanlike neck which is almost as long as the body and tail together, consists of from twenty to forty vertebræ. The tail is relatively much shorter than that of the Ichthyosaur, and there is, Professor Owen observes, an obvious reason, for the curtailment of this part of the animal; because the length and mobility of the neck of the Plesiosaur renders a special development of the tail for producing the lateral movements of the head unnecessary.

In the anterior or pectoral extremities the digits never exceed the metacarpal bones, which are five in number. The first, or radial digit, corresponding with the thumb, has generally three; the second six or seven, the third eight or nine, the fourth eight,

and the fifth six phalanges. Professor Owen, who gives these numbers, adds that there can be little doubt that they were enveloped, like the paddles of Cetacea, in a common sheath of integument, and that, from the natural curve of the digits, the paddles of the Plesiosaur must have had a more elegant and tapering form, and have possessed greater flexibility than those of the modern whales.

The posterior or pelvic extremities almost always equal, and sometimes, as in Plesiosaurus macrocephalus, exceed the anterior extremity, but they closely correspond with them in their radiated appendages. The five metatarsals and their digits, Professor Owen observes, correspond in structure with those of the forepaddle. The first or tibial metatarsal, he tells us, supports three phalanges, the second five, the third eight or nine, the fourth eight, and the fifth six phalanges. The structure of the bones of this extremity indicate, in Professor Owen's opinion, that the hind paddle had a freer inflection forwards or upon the tibia, than in the opposite direction; and he thinks that it may have given a compound motion to the propelling stroke of the paddle, similar to that which in skilful rowing is termed "feathering the oar." He further remarks, that the articular extremities of the phalanges of both the fore and hind paddles are sub-concave, with an irregular surface, indicating that they were joined by ligaments or fibro-cartilage, and not by a synovial membrane.

But what were the habits of this chimæra-like creature? The best answer will be given by the Rev. W. Conybeare, who thus infers those of 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 a 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 seaweed, and raising its nostrils to a level with the surface from a considerable depth, may have found a secure retreat from the assault of dangerous enemies; while the length and flexibility 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.”

Professor Buckland is of opinion that the tail, being comparatively short, could not have been used like the tail of fishes, as an instrument of rapid impulsion in a forward direction; but was probably employed more as a rudder to steer the animal when swimming on the surface, or to elevate or depress it in ascending and descending through the water. The same consequence as to slowness of motion, would, he thinks also, follow from the elongation of the neck to so great a distance in front of the anterior paddles. The total number of vertebræ in the entire column was, he observes, about ninety. From all these circumstances, Dr. Buckland infers that this animal, although of considerable size, had to seek its food as well as its safety, chiefly by means of artifice and concealment.

No less than sixteen species are enumerated by Professor Owen, -one described by Cuvier, two by Conybeare, and the rest by himself.

The period of existence of these enaliosaurians, extended through the whole of the oolitic range, including the lias and oolite of the Wealden and chalk formations. The chalk marl appears to be the most recent deposit where they have been found: they occur also in the gault.

"these

Their name was legion. To say nothing of the bones which testify to their numbers, the petrified remains of their digested food put the question of their numerical force out of doubt. "On the shore at Lyme Regis," says Dr. Buckland, coprolites are so abundant, that they lie like potatoes scattered in the ground; still more common are they in the lias of the Estuary of the Severn, where they are similarly disposed in strata of many miles in extent, and mixed so abundantly with teeth and rolled fragments of the bones of reptiles and fishes, as to show that this region, having been the bottom of an ancient sea, was for a long period the receptacle of the bones and focal remains of its inhabitants. The occurrence of coprolites is not, however, peculiar to the places just mentioned; they are found in greater or less abundance throughout the lias of England; they occur also in strata, of all ages, that contain the remains of carnivorous reptiles, and have been recognised in many and distant climates both of Europe and America.'

The sea in which these extinct monsters gambolled, must have been not unlike that of the present day, especially in tropical climates. That the medium was capable of transmitting light in