bræ constructed after the manner of fishes, had they been furnished with legs instead of paddles, could not have moved on land without injury to their backs.*

Ribs.

The ribs were slender, and most of them bifurcated at the top they were also continuous along the whole vertebral column, from the head to the pelvis, (see Plates 7, 8, 9 ;) and in this respect agree with the structure of modern Lizards. A considerable number of them were united in front across the chest: their mode of articulation may be seen in Pl. 14. The ribs of the right side were united to those of the left, by intermediate bones, analogous to the cartilaginous intermediate and sternal portions of the ribs in Crocodiles; and to the bones which, in the Plesiosaurus, form what Mr. Conybeare has called the sterno-costal arcs. (See Pl. 17.) This structure was probably subservient to the purpose of introducing to their bodies an unusual quantity of air; the animal by this means being enabled to remain long beneath the water, without rising to the surface for the purpose of breathing.†

* Sir E. Home has farther remarked a peculiarity of the spinal canal, which exists in no other animals; the annular part (Pl. 12, D a. and E a.) being neither consolidated with the body of the vertebra, as in quadrupeds; nor connected by a suture, as in Crocodiles; but remaining always distinct, and articulating by a peculiar joint, resembling a compressed oval ball and socket joint, (D g. and E g.) And Mr. Conybeare adds, that this mode of articulation co-operates with the cup-shaped form of the intervertebral joints, in giving flexibility to the vertebral column, and assisting its virbratory motions; for, had these parts been consolidated, as in quadrupeds, their articulating processes must have locked the whole column together, so as to render such a motion of its parts impossible; but by means of this joint every part yields to that motion. The tubercle by which the transverse apophysis of the head of the rib articulates with the vertebra, is seen at d.

The sterno-costal ribs probably formed part of a condensing apparatus, which gave these animals the power of compressing the air within its lungs,

Sternum.

To a marine animal that breathed air, it was essential to possess an apparatus whereby its ascent and descent in the water may have been easily accomplished; accordingly we find such an apparatus, constructed with prodigious strength, in the anterior paddles of the Ichthyosaurus; and in the no less extraordinary combination of bones that formed the sternal arch, or that part of the chest, on which these paddles rested. Pl. 12, Fig. 1.

It is a curious fact, that the bones composing the sternal arch are combined nearly in the same manner as in the Ornithorhynchus* of New Holland; which seeks its food

before they descended beneath the water. In the Lond. and Edin. Phil. Mag. Oct. 1833, Mr. Faraday has noticed a method of preparing the organs of respiration in man, so as considerably to extend the time of holding the breath in an impure atmosphere; or under water, as practised by pearl-fishers, and illustrated by experiments of Sir Graves C. Houghton. If a person inspires deeply, and ceasing with his lungs full of air, holds his breath as long as he is able, the time during which he can remain without breathing will be double, or more than double, that which he could do if he held his breath without such deep inspiration. When Mr. Brunel, jun. and Mr. Gravatt descended into a diving-bell to examine the hole where the Thames had broken into the tunnel at Rotherhithe, at the depth of about thirty feet of water, Mr. Brunel, having inspired deeply the compressed air within the diving-bell, descended into the water below the bell; and found that he could remain twice as long under water, going into it from the diving-bell, at that depth, as he could under ordinary circumstances.

Mr. Gravatt has also informed me that he is able to dive and remain three minutes under water, after inflating his lungs with the largest possible quantityof common air, by a succession of strong and rapid inspirations, and immediately compressing the lungs thus filled with air, by muscular exertion, and contraction of the chest, before he plunges into the water. By this compression of the lungs, the specific gravity of the body is also increased, and the descent is consequently much facilitated.

All these advantages were probably united in the mode of respiration of the Ichthyosaurus, and also in the Plesiosaurus.

* In this anomalous animal the Ornithorhynchus or Platypus, we have

at the bottom of lakes and rivers, and is obliged, like the Ichthyosaurus, to be continually rising to the surface to breathe air.*

Here then we have a race of animals that became extinct at the termination of the secondary series of geological formations, presenting, in their structure, a series of contrivances, the same in principle, with those employed at the present day to effect a similar purpose in one of the most curiously constructed aquatic quadrupeds of New Holland.†

Paddles.

In the form of its extremities, the Ichthyosaurus deviates from the Lizards, and approaches the Whales. A large

a quadruped clothed with fur, having a bill like a duck, with four webbed feet, suckling its young, and most properly ovoviviparous the male is furnished with spurs. See Mr. R. Owen's Papers on the Ornithorhynchus Paradoxus, in the Phil. Trans. London, 1832, Part II. and 1834, Part II. See also Mr. Owen's Paper on the same subject in Trans. Zool. Soc. Lond. Part III. 1835, in which he points out many approximations in the generative and other systems of this animal to the organization of reptiles.

* In both these animals there is superadded to the ordinary type of bones in quadrupeds, an enlargement of the coracoid bone (c,) and a peculiar form of sternum, resembling the furcula of birds. In Pl. 12, Fig. 1, a. represents the peculiar sternum or furcula; b. b. the clavicles; c. c. the coracoid bones: d. d. the scapula; e. e. the humeri; f. g. the radius and ulna. At Fig. 2, the same letters are attached to the corresponding bones of the Ornithorhynchus.

The united power of all these bones imparts to the chest and paddles peculiar strength for an unusual purpose; not so much to effect progressive motion (which, in the Ichthyosaurus, was produced with much greater facility and power by the tail,) as to ascend and descend vertically in quest of air and food.

+ The Echidna, or spiny Ant-eater, of New Holland, is the only known land quadruped that has a similar furcula and clavicles. As this animal feeds on Ants, and takes refuge in deep burrows, this structure may be subsidiary to its great power of digging. A cartilaginous rudiment of a furcula occurs also in the Dasypus; and seems subservient to the same purpose.

animal, moving rapidly through the sea, and breathing air, must have required great modification of the fore-leg and foot of the Lizard, to fit it for such cetaceous habits. The extremities were to be converted into fins instead of feet, and as such we shall find them to combine even a still greater union of elasticity with strength, than is presented by the fin or paddle of the Whale. Plate 12, Fig. 1, shows the short and strong bones of the arm (e,) and those of the fore-arm (f, g;) and beyond these the series of polygonal bones that made up the phalanges of the fingers. These polygonal bones vary in number in different species, in some exceeding one hundred; they differ also in form from the phalanges both of Lizards and Whales; and derive, from their increase of number, and change of dimensions, an increase of elasticity and power. The arm and hand thus converted into an elastic oar or paddle, when covered with skin, must have much resembled externally the undivided paddle of a Porpoise or Whale. The position also of the paddles on the anterior part of the body was nearly the same; to these were superadded posterior extremities, or hind fins, which are wanting in the cetacea, and which possibly make compensation for the absence of their flat horizontal tail: these hind paddles in the Ichthyosaurus are nearly by one half smaller than the anterior paddles.*

Mr. Conybeare remarks, with his usual acumen, that "the reasons of this variation from the proportions of the posterior extremities of quadrupeds in general, are the same which lead to a similar diminution of the analogous parts in Seals, and their total disappearance in the cetacea, namely, the necessity of placing the centre of the organs of motion, when acting laterally, before the centre of gravity. For the same reason, the wings of birds are placed in the fore part of their body, and the centre of the moving forces given to

* In the Ornithorhynchus, also, the membraneous expansion, or web of the hind feet, is very much less than that on the fore-foot.

ships by their sails, and to steam-boats by their paddles, is similarly placed. The great organ of motion in fishes, the tail, is indeed posteriorly placed, but this by its mode of action generates a vis a tergo, which impels the animal straight forwards, and does not therefore operate under the same conditions with organs laterally applied." G. T. V. 5, p.

579.

I shall conclude this detailed review of the peculiarities of one of the most curious, as well as the most ancient, among the many genera of extinct reptiles presented to us by Geology, with a few remarks on the final causes of those deviations from the normal structure of its proper type, the Lizard; under which the Ichthyosaurus combines in itself the additional characters of the fish, the Whale, and Ornithorhynchus. As the form of vertebræ by which it is associated with the class of fishes, seems to have been introduced for the purpose of giving rapid motion in the water to a Lizard inhabiting the element of fishes; so the farther adoption of a structure in the legs, resembling the paddles of a Whale, was superadded in order to convert these extremities into powerful fins. The still farther addition of a furcula and clavicles, like those of the Ornithorhynchus, offers a third and not less striking example of selection of contrivances, to enable animals of one class to live in the element of another class.

If the laws of co-existence are less rigidly maintained in the Ichthyosaurus, than in other extinct creatures which we discover amid the wreck of former creations, still these deviations are so far from being fortuitous, or evidencing imperfection, that they present examples of perfect appointment and judicious choice, pervading and regulating even the most apparently anomalous aberrations.

Having the vertebræ of a fish, as instruments of rapid progression; and the paddles of a Whale, and sternum of an Ornithorhynchus, as instruments of elevation and depression; the reptile Ichthyosaurus united in itself a combination VOL. I.-13