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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.*

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.

Eyes.

The enormous magnitude of the eye of the Ichthyosaurus (Pl. 10, Fig. 1, 2,) is among the most remarkable peculiarities in the structure of this animal. From the quantity of light 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 (Pl. 10, Fig. 3.) This compound circle of bony plates, does not occur in fishes; but is found in the eyes of many birds.†

* In Pl. 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. c, 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 Iehthyosaurus, in which the lower 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.

The bony sclerotie of the Ichthyosaurus approaches to the form of the bony circle in the eye of the Golden Eagle (Pl. 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

as well as of Turtles, Tortoises, and Lizards; and in a less degree in Crocodiles. (Pl. 10. Figs. 4, 5, 6.)

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; but 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 night, 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 appara

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 (Pl. 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 may be 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.” Yarrel on the Anatomy of Birds of Prey, Zool. Journal, v. 3, p. 188.

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.

tus of bony plates, was to give strength to the surface of so large an eye-ball, enabling it the better to resist the pressure of deep water, to which it must often have been exposed; it would also have protected this important organ from injury by the waves of the sea, to which an eye, sometimes larger than a man's head, must frequently have been subject, wher the nose was brought to the surface, for the necessary purpose of breathing air: the position of the nostrils, close to the anterior angle of the eye, rendered it impossible for the Ichthyosaurus to breathe without raising its eye to the surface of the water..

Jaws.

The Jaws of the Ichthyosauri, like those of Crocodiles and Lizards, which are all more or less elongated into projecting beaks, are composed of many thin plates, so arranged as to combine strength with elasticity and lightness, in a greater degree than could have been effected by single bones, like those in the jaws of Mammalia. It is obvious that an under jaw so slender, and so much elongated as that of a Crocodile or Ichthyosaurus, and employed in seizing and retaining the large and powerful animals which formed their prey, would have been comparatively weak and liable to fracture if composed of a single bone. Each side of the lower jaw was therefore made up of six separate pieces, set together in a manner that will be best understood by reference to the Figures in Pl. 11.*

*These figures are selected from various plates by Mr. Conybeare and Mr. De la Beche. Fig. 1 is a restoration of the entire head of an Ichthyosaurus, in which each component bone is designated by the letters appropriated by Cuvier to the equivalent bones in the head of the Crocodile. In the lower jaw, u, marks the dental bone; v,, the angular bone; x, superangular or coronoid; y, articular bone; z, complimentary; §, opercular. Fig. 2, is part of an under jaw of an Ichthyosaurus, showing

This contrivance in the lower jaw, to combine the greatest elasticity and strength with the smallest weight of materials, is similar to that adopted in binding together several parallel plates of elastic wood, or steel, to make a crossbow; and also in setting together thin plates of steel in the springs of carriages. As in the carriage spring, or compound bow, so also in the compound jaw of the Ichthyosaurus, the plates are most numerous and strong, at the parts where the greatest strength is required to be exerted; and are thinner and fewer towards the extremities, where the service to be performed is less severe. Those who have witnessed the shock given to the head of a Crocodile, by the act of snapping together its thin long jaws, must have seen how liable to fracture the lower jaw would be, were it composed of one bone only on each side: a similar inconvenience would have attended the same simplicity of structure in the jaw of the Ichthyosaurus. In each case, therefore, the splicing and bracing together of six thin flat bones of unequal length, and of varying thickness, on both sides of the lower jaw, affords compensation for the weakness and risk of fracture, that would otherwise have attended the elongation of the snout.

Mr. Conybeare points out a farther beautiful contrivance in the lower jaw of the Ichthyosaurus, analogous to the cross bracings lately introduced in naval architecture, (see Pl. 11, Fig. 2.*)

the manner in which the flat bones, v, x, u, are applied to each other, towards the posterior part of the jaw. Figs. 3, 4, 5, 6, 7, show the manner in which these bones overlap, and lock into each other, at the transverse sections, indicated by the lines immediately above them in Fig. 2. Fig. 8, shows the composition of the bones in the lower jaw, as seen from beneath.

* The coronoid bone, (x) is interposed between the dental, (u,) and opercular (&,) its fibres have a slanting direction, whilst those of the two latter bones are disposed horizontally; thus, the strength of the part is greatly increased by a regular diagonal bracing, without the least addi

Vertebra.

The vertebral column in the Ichthyosaurus was composed of more than one hundred joints; and although united to a head nearly resembling that of a Lizard, assumed, in the leading principles of its construction, the character of the vertebræ of fishes. As this animal was constructed for rapid motion through the sea, the mechanism of hollow vertebræ, which gives facility of movement in water to fishes, was better calculated for its functions than the solid vertebræ of Lizards and Crocodiles. (See Plate 12, A. and B.). This hollow conical form would be inapplicable to the vertebræ of land quadrupeds, whose back, being nearly at right angles to the legs, requires a succession of broad and nearly flat surfaces, which press with considerable weight against each other. It is quite certain, therefore, that such large and bulky creatures as the Ichthyosauri, having their verte-.

tion of weight or bulk; a similar structure may be noticed in the overlapping bones of the heads of fish, and in a less degree, in those of Turtles.— Geol. Trans. Lond. Vol. V. p. 565, and Vol. I. N. S. p. 112.

*The sections of the vertebræ of a fish (A c. c.) present two hollow cones, united at their apex in the centre of each vertebra, in the form of an hour-glass; but the base of each cone, (b. b.) instead of terminating in a broad flat surface, like the base of the hour-glass, is bounded by a thin edge, like the edge of a wine-glass, and by this alone touches the corresponding edge of the adjacent vertebra. Between these hollow. vertebræ, a soft and flexible intervertebral substance, in the form of a double solid cone (e. e.) is so placed that each hollow cone of bone plays on the cone of elastic substance contained within it, with a motion in every direction; thus forming a kind of universal joint, and, giving to the entire column great strength, and power of rapid flexion in the water. But as the inflections in the perpendicular direction are less necessary. than in the lateral, they are limited by the overlapping, or contiguity of the spines.

This mode of articulation gives mechanical advantage to animals like fishes, whose chief organ of progressive motion is the tail; and the weight of whose bodies being always suspended in water, creates little or no pressure on the edges, by which alone the vertebræ touch each other.

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