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EXPLANATION OF PLATE 11. 23
Plate 11. V. I. p. 138.
1. Side View of the head of an Ichthyosaurus, marking by corresponding letters, the analogies to Cuvier's figures of the same bones in the head of the Crocodile. (Conybeare.) 2. Posterior part of a lower jaw of Ichthyosaurus communis, in the Oxford Museum. (Conybeare.) 3–7, Sections presented by the component bones of Fig. 2 in fractured parts above each section. (Conybeare.) 8. View of the lower Jaw of Ichthyosaurus seen from
codiles; 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 unfrequently 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.
beneath, exhibiting the course of its over-lapping bones. (Conybeare.) A. Tooth of a Crocodile, showing the incipient absorption of the hollow cone which forms its base, from the effect of pressure of a new tooth rising beneath. (Conybeare.) B. Similar effects shown in the transverse section of the upper and lower jaws of an Ichthyosaurus. (Cuvier.) C. Example of the same kind of absorption produced by the pressure of a new tooth, on the base of an older tooth in the jaw of Ichthyosaurus. (Conybeare.)
PLATE 12. V. I. p. 142.
1. Sternal Arch and Paddles of Ichthyosaurus. See V. I. p. 182, Note. (Home.)
2. Sternal Arch of Ornithorhynchus. (Home.)
3, 4, 5, 6. Occipital and Cervical Bones of Ichthyosaurus, from the Lias at Lyme Regis.” (Original.)
* Sir Philip de Malpas Grey Egerton has pointed out some beautiful examples, hitherto unnoticed, in the Atlas and cervical Vertebrae of Ichthyosauri, of peculiar mechanical contrivances to support and regulate the movements of their enormous heads. (See Lond. and Edin. Phil. Mag. Nov. 1835. p. 414.) Fig. 3, a represents the Basilar portion of thc Occipital bone of a very large and aged Ichthyosaurus, from the Lias of Lyme Regis, (scale one-eighth.) The nearly hemispherical process (a) articulated with a comparatively shallow socket in front of the Atlas, (4. a.) and this ball and socket, or universal joint, gave freedom of motion and support to a weighty head. Fig. 4. Atlas and Axis of a very young Ichthyosaurus, (two-thirds of nat. size.) These bones adhere together by two nearly flat surfaces, admitting of the least flexure of any of the Vertebrae in the whole body, but giving the greatest strength to that part of the Column, where strength rather than flexure was required. On the inferior margins of the Atlas and Axis and third cervical vertebra, are triangular facets articulating with three strong wedgeshaped sub-vertebral bones (c) hitherto undescribed.
ExPLANATION OF PLATEs 12. 13. 25
A. Hollow conical Vertebrae of a fish. (Original.)
B. C. E. Vertebrae of Ichthyosaurus. "See note, V. I. p. 212. (Home and Conybeare.)
D. a. g. E. a. g. Spinous processes, showing the peculiar articulation of their annular portions, with the Wertebrae, to be adapted to increase the flexibility of the spine. See Note, V. I. p. 134. (Home.)
PLATE 13. V. I. p. 149. Skeleton of a small Ichthyosaurus, from the Lias at
Fig. 4, b. Oblique triangular facet on the lower margin of the front of the Atlas; this facet articulated with the first sub-vertebral wedge, placed between the Atlas and Occiput. Between the Atlas and Axis, the two sub-vertebral facets formed a triangular cavity for the reception of a second wedge (Fig. 4. c.) and a similar, but smaller cavity received another wedge of the same kind, between the Axis and third Vertebra. This third wedge gave less support to the head, and admitted of more extensive motion than the second. All these three wedge-shaped bones are seen nearly in their natural position in a specimen from Lyme Regis, in the Collection of Sir P. G. Egerton. Fig. 4. First sub-vertebral wedge, auxiliary to the anterior cavity of the Atlas, in completing the articulating socket for the basilar process of the Occiput (3.a.) 4. a. Crescent-shaped front of the first sub-vertebral wedge. 4'. b. Head of the same Wedge. 4'. c. Obtuse apex of the same, articulating with the triangular frontal facet of the Atlas (4. b.) In young animals this frontal facet is nearly smooth and flat; in older animals (3.b.) it is rugged and furrowed. This articulation must have given to the first sub-vertebral wedge great power as a stay or prop, to resist the downward pressure of the head, at the same time facilitating the rotatory movements of the Occipital bone. Fig. 4. c. Second sub-vertebral wedge articulating with the triangular cavity formed by the marginal facets of the Atlas and Axis. This second Wedge acted as a strong prop supporting firmly the lower portion of the Atlas, and at the same time admitting the small amount of motion here required. c. Head of the sub-vertebral wedge (c) strengthened by a projecting boss of solid bone.
Lyme Regis, presented to the Oxford Museum by Viscount Cole, enclosing within its ribs scales, and digested bones of Fishes, in the state of Coprolite. This coprolitic mass seems nearly to retain the form of the stomach of the animal. c, Coracoid bone. d, Scapula, e, Humerus. f. Radius, g, Ulna. (Scharf. Original.)
PLATE 14. V. I. p. 152. Skeleton of the Trunk of a small Ichthyosaurus in the
Fig. 5. Nearly flat articular surface of (probably) the third cervical vertebra of the same large Individual as Fig. 3. This surface of the bone has only a small cylindrical depression at its centre, instead of the deep, conical cup of the more flexible vertebrae, C. B. E. Near its upper margin is a wedge-shaped elevation (b) and near the inferior margin, a notch or furrow (a.) These salient and re-entering portions articulated with corresponding depressions and projections on the surface of the adjacent vertebra, and acted as pivots, admitting a limited amount of lateral vibrations, and at the same time preventing any slip, or dislocation. Fig. 6. Concave surface of Fig. 5.; the wedge-shaped projection near its lower margin (a) must have articulated with a corresponding groove or depression on the front of the vertebra adjacent to it, like that at (Fig. 5 a.) As one surface only of these vertebrae had a conical cavity, the intervertebral substance must have formed a single cone, admitting in the neck but half the amount of motion, that the double cones of intervertebral matter allowed to the dorsal and caudal vertebrie, (C. B. E.) where greater flexure was required, to effect progressive motion by vibrations of the body and tail. These dispositions of the articulating facets of the cervical vertebræ, acting in conjunction with the three sub-vertebral wedges before described, afford an example of peculiar provisions in the neck of these gigantic Reptiles, to combine a diminished amount of flexure in this part, with an increased support to their enormous heads, It is probable that every species of Ichthyosaurus had peculiar variations in the details of the cervical vertebrae, and subvertebral wedges, and that in each species these variations were modified by age. In the Gavial Mr. Mantell has recently observed that the first caudal vertebra is doubly conver, like the last cervical vertebra in Turtles. These peculiar contrivances give to the animals in which they occur increased flexibility of the Tail and Neck.
Explanation of plate 15. 27
Oxford Museum, from the Lias at Lyme Regis, containing within the ribs, a coprolitic mass of digested bones, interspersed with scales of fishes. a, Furcula. b, Clavicle, c, Coracoid bone. d, Scapula. e, Humerus. (Fisher. Original.)
PLATE 15. W. I. p. 147.)
The specimens are all of the natural size except where the figures denote otherwise. (Original.)
1 and 2. Intestines of the two most common English
species of Dog-Fishes, injected with Roman cement. The vascular structure, which is still apparent in
the desiccated membrane, resembles the impressions on the surface of many Coprolites.
3. Coprolite from the Lias at Lyme, exhibiting the spiral folding of the plate of digested bone, and impressions of the intestinal vessels and folds upon its surface. (See Note, V. I. p. 152. et seq.) t
3. Magnified scale of Pholidophorus limbatus, embedded in the surface of the Coprolite, Fig. 3. This scale is one of those that compose the lateral line, by which a tube passes to convey mucus, from the head, along the body of fishes; a. is the hook, on the superior margin, which is received by a depression on the inferior margin of the scale above it, corresponding with b.; c. is the serrated edge of the posterior margin, perforated at e. for the passage of the mucous duct; d. is a tube on the interior surface of the scale to carry and protect the mucous duct. (See note V. I. p. 150.)
3". Exterior of the scale 3'.; the same parts are represented by the same letters; the larger portion is covered with enamel; the smaller portion next d. is the bony root forming the anterior margin of the scale.