Geology and Mineralogy considered with reference to Natural Theology/Plate 10

From Wikisource
Jump to navigation Jump to search
Geology and Mineralogy considered with reference to Natural Theology, plate 10.png

Plate 10. V. I. pp. 135, 137.

1. Head of Ichthyosaurus platyodon, in the British Museum, from the Lias at Lyme Regis, copied from Sir E. Home's figure in the Phil. Trans. 1814.
2. Copied from Mr. Conybeare's figure, (in the Geol. Trans. Lond. O. S. PI. XL. Fig. 11.) showing the analogies between the bones of the head of Ichthyosaurus, and those which Cuvier has marked by corresponding letters in his figure of the head of the Crocodile.
3. Two of the bony plates in the sclerotic coat of the Eye of Ichthyosaurus platyodon.
4. Circle of bony plates in the Eye of the snowy Owl. (Yarrel.)
5. Circle of similar plates in the Eye of the golden Eagle, (Yarrel.)

6. Front view of bony plates in the Eye of an Iguana.

7. Profile of the same.

8. Two of the fourteen component scales of the same.

I owe these three last figures to the kindness of Mr. Allis of York.

A 1, 2, 3, 4. Petrified portions of the skin of a small Ichthyosaurus, from the Lias of Barrow on Soar, Leicestershire, presented to the Oxford Museum, by the Rev. Robert Gutch, of Segrave. (Original.)
In Fig. 1; a, b, c, d, are portions of ribs, and e, f, g, h, are fragments of sterno-costal bones (nat. size.)

The spaces between these bones, are covered with the remains of skin; the Epidermis being represented by a delicate film, and the Rete mucosum by fine threads of white Carbonate of Lime; beneath these the Corium, or true skin, is preserved in the state of dark Carbonate of Lime, charged with black volatile matter, of a bituminous and oily consistence.

2. Magnified representation of the Epidermis and Rete mucosum. The fine superficial lines represent the minute wrinkles of the Epidermis, and the subjacent larger decussating lines, the vascular net-work of the Rete mucosum.
In Fig. 3, the Epidermis exhibits a succession of coarser and more distant folds or wrinkles overlying the mesh-work of the Rete mucosum.
In Fig. 4, the Epidermis has perished, and the texture of the fine vessels of the Reta mucosum is exhibited in strong relief, over the black substance of the subjacent Corium, in the form of a net-work of white threads.[1]

  1. Nothing certain has hitherto been known respecting the dermal covering of the Ichthyosauri; it might have been conjectured that these reptiles were incased with horny scales, like Lizards, or that their skin was set with; dermal bones, like those on the back of Crocodiles; but as the horny scales of Fishes, and dermal bones of Crocodilean animals are preserved in the same Lias with the bones of Ichthyosauri, we may infer that if the latter animals had been furnished with any similar appendages, these would also have been preserved, and long ere this discovered, among the numerous remains that have been so assiduously collected from the Lias. They would certainly have been found in the case of the individual now before us, in which even the Epidermis, and vessels of the Rete Mucosum have escaped destruction.

    Similar black patches of petrified skin are not infrequently found attached to the skeletons of Ichthyosauri from Lyme Regis, but no remains from any other soft parts of the body have yet been noticed.

    The preservation of the skin shows that a short interval only elapsed between the death of the animal, and its interment in the muddy sediment of which the Lias is composed.

    Among living reptiles, the Betrachians afford an example of an order in which the skin is naked, having neither scales nor dermal bones.

    In the case of Lizards and Crocodiles, the scaly, or bony coverings protect the skin from injury by friction against the hard substances with which they are liable to come into contact upon the land; but to the Ichthyosauri which lived exclusively in the sea, there would seem to have been no more need of the protection of scales or dermal bones, than to the naked skin of the Cetacea.

    In the case of Plesiosauri also, the non-discovery of the remains of any dermal appendages with the perfect skeletons of animals of that genus, leads to a similar inference, that they too had a naked skin. The same negative argument applies to the flying Reptile Family of Pterodactyles.