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Nearly at the head of the surprising discoveries, which have been made relating to the family of Saurians, we may rank the remains of many extraordinary species, which inhabited the sea; and which present almost incredible combinations of form, and structure; adapting them for modes of life that do not occur among living reptiles. These remains are most abundant throughout the lias and oolite formations of the secondary series.* In these deposites we find not only animals allied to Crocodiles, and nearly approaching to the Gavial of the Ganges; but also still more numerous gigantic Lizards, that inhabited the then existing seas and estuaries.
Some of the most remarkable of these reptiles have been arranged under the genus Ichthyosaurus, (or Fish Lizard,) in consequence of the partial resemblance of their vertebrae to those of fishes. (See Plate 1, Fig. 51, and Plates 7,8,9.) If we examine these creatures with a view to their capabilities of locomotion, and the means of offence and defence which their extraordinary structure afforded to them; we shall find combinations of form and mechanical contrivances which are now dispersed through various classes and orders of existing animals, but are no longer united in the same genus. Thus, in the same individual, the snout of a Porpoise is combined with the teeth of a crocodile, the head of a Lizard with the Vertebrae of a fish, and the sternum of an Ornithorhynchus with the paddles of a whale. The general outline of an Ichthyosaurus must have most nearly resembled the modern Porpoise, and Grampus. It had four broad feet, or paddles, (PI. 7,) and terminated behind in a long and powerful tail. Some of the largest of these reptiles must have exceeded thirty feet in length.
* The chief repository in which these animals have been found is the lias, at Lyme Regis; bat they abound also along the whole extent of this formation throughout England, e. g. from the coast of Dorset, through Somerset and Leicestershire, to the coast of Yorkshire: they are found also in the lias of Germany and France. The range of the genus Ichthyosaurus seems to have begun with the Muschelkalh, and to have extended through the whole of the oolitic period into the cretaceous formation. The most recent stratum in which any remains of this genus have yet been found is the chalk marl at Dover, where they have been discovered by Mr. Mantell: I have found them in the gault, near Benson, Ozon.
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There are seven or eight known species of the genus Ichthyosaurus, all agreeing with one another in the general principles of their construction, and the possession of those peculiar organs, in which I shall endeavour to point out the presence of mechanism and contrivance, adapted to their habits and state of life. As it will be foreign to our purpose to enter on details respecting species, I shall content myself with referring to the figures of the four most common forms (Plates 7, 8, 9.)
* PI. 7, is a large and nearly perfect specimen of the Ichthyosaurus Platyodon, from the lias at Lyme Regis, being one of the splendid scries of Saurians, purchased in 1834 of Mr. Hawkins by the British Museum. Tortious of the paddles, and many lost fragments, are restored from the corresponding parts which are preserved; a few vertebra?, and the extremity of the tail are also restored conjecturally. Beautiful and accurate lithographed figures of this specimen, and of the greater part of this collection , are published in Mr. Hawkins's Memoirs of Ichthyosauri and Plesiosauri, London, 1834. PI. 8. Fig. 1, is a small specimen of the Ichthyosaurus Communis, from the lias at Lyme Regis, belonging to the Geol. Soc. of London. PI. 8, Fig. 2, a small Ichthyosaurus Intermedins, from the lias at Lyme Regis belonging to Sir Astley Cooper. PI. 9, Fig. I, an Ichchyosaurus Tenuirostris, from the lias of Street, near Glastonbury, in the collection of Rev. D. Williams. Fig. 2 is the continuation of the tail, and Fig. 3, the reverse of the head. The teeth in this species are small, and in due proportion to the slender character of the snout.
The head, which in all animals forms the most important and characteristic part, (see PL 10, Figs. 1,2,) at once shows that the Ichthyosauri were Reptiles, partaking partly of the characters of the modern Crocodiles, but more allied to Lizards. They approach nearest to Crocodiles in the form and arrangement of their teeth. The position of the nostril is not, as in Crocodiles, near the point of the snout; it is set, as in Lizards, near the anterior angle of the orbit of the eye. The most extraordinary feature of the head, is the enormous magnitude of the eye, very much exceeding that of any living animal.* The expansion of the jaws must have been prodigious; their length in the larger species, (Ichthyosaurus Platyodon,) sometimes exceeding six feet; the voracity of the animal was doubtless in proportion to its powers of destruction. The neck- was short, as in fishes.
The teeth of the Ichthyosaurus (PI. 11, B, C,) are conical, and much like those of the Crocodiles, but considerably more numerous, amounting in some cases to a hundred and eighty; they vary in each species; they are not enclosed in deep and separate sockets, as the teeth of Crocodiles, but are ranged in one long continuous furrow, (PI. 11, B, C,) of the maxillary bone, in which the rudiments of a separation into distinct alveoli may be traced in slight ridges extending between the teeth, along the sides and bottom of the furrow. The contrivance by which the new tooth replaces the old one, is very nearly the same in the Ichthyosauri as in the Crocodiles (PL 11, A, B, C;) in both, the young tooth begins its growth at the base of the old tooth, where, by pressure on one side it causes first a partial absorption of the base, and finally a total removal of the body of the older tooth, which it is destined to replace.*
* Jn the collection of Mr. Johnson, at Bristol, is a skull of Ichthyosaurus Platyodon, in which the longer diameter of the orbital cavity measures fourteen inches.
As the predaceous habits of the Ichthyosauri exposed them, like modern Crocodiles, to frequent loss of their teeth, an abundant provision has in each case been made for their continual renewal.
The enormous magnitude of the eye of the Ichthyosaurus (PI. 10, Fig. 1, 2,) is among the most remarkable peculiarities in the structure of this animal. From the quantity of Kght admitted in consequence of its prodigious size, it must have possessed very great powers of vision; we have also evidence that it had both microscopic and telescopic properties. We find on the front of the orbital cavity in which this eye was lodged, a circular series of petrified thin bony plates, ranged around a central aperture, where once was placed the pupil; the form and thickness of each of these plates very much resembles that of the scales of an artichoke (PI. 10, Fig. 3.) This compound circle of bony plates, does not occur in fishes; but is found in the eyes of many birds.f as well as of Turtles, Tortoises, and Lizards; and in a less degree in Crocodiles. (PI. 10. Figs. 4, 5, 6.)
* In PI. 11. Fig. A, shows the manner in which the older tooth in the Crocodile becomes absorbed, by pressure of a younger tooth rising within the cavity of its hollow base. Fig. o, represents a transverse section of the left side of the lower jaw of an Ichthyosaurus, showing two teeth in their natural place, within the furrows of the jaw; the younger tooth, by lateral pressure, has caused absorption of the inside portion of the base of the older tooth. Fig, B, represents a transverse section of the entire snout of an Ichthyosaurus, in which the tower jaw exhibits on both sides, a small tooth (a) which has caused partial absorption of the base of the larger tooth (e.) In the upper jaw, the bases of two large teeth (d, d,} are seen in their respective furrows.
t The bony sclerotie of the Ichthyosaurus approaches to the form of the bony circle in the eye of the Golden Eagle (PI. 10, Fig. 5;) one of its uses in each case being to vary the sphere of distinct vision, in order to descry their prey at long or short distances. These bony plates also assist to maintain the prominent position of the front of the eye, which is so remarkable in birds. In Owls, whose nocturnal habits render distant vision impossible, Mr. Yarrel observes, that the bony circle (PI. 10, Fig. 4,) is concave, and elongated forwards, so that the front of the eye is placed at the end of a long tube, and thus projects beyond the loose and downy feathers of the head; he adds; "The extent of vision enjoyed by the Falcons is probably denied to the Owls, but their more spherical lens and corresponding cornea give them an intensity better suited to the opacity of the medium in which they are required to exercise this power. They maybe compared to a person near-sighted, who sees objects with superior magnitude and brilliancy when within the prescribed limits of his natural powers of vision, from the increased angle these objects subtend." Yan-el on the Anatomy of Birds of Prey, Zool. Journal, v. 3, p. 188.
In living animals these bony plates are fixed in the exterior or sclerotic coat of the eye, and vary its scope of action, by altering the convexity of the cornea: by their retraction they press forward the front of the eye and convert it into a microscope; in resuming their position, when the eye is at rest, they convert it into a telescope. The soft parts of the eyes of the Ichthyosauri have of course entirely perished; hut the preservation of this curiously constructed hoop of bony plates, shows that the enormous eye, of which they formed the front, was an optical instrument of varied and prodigious power, enabling the Ichthyosaurus to descry its prey at great or little distances, in the obscurity of nigh:, and in the depths of the sea; it also tends to associate the animal, in which it existed, with the family of Lizards, and exclude it from that of fishes.*
A farther advantage resulting from this curious apparr.
* There are analogous contrivances for the purpose of resisting pressure, and maintaining the form of the eye in fishes, by the partial or total ossification of the exterior capsule; but in fishes, this ossification is usually simple, though carried to a different extent in different species; and the bone is never divided transversely into many plates, as in Lizards, and Birds; these capsules of the eye are often preserved in the heads of fossil fishes: they abound in the London clay; and occasionally occur in chalk.