Geology and Mineralogy considered with reference to Natural Theology/Plate 11

by William Buckland

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 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.^[1] (Original.)

1. 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 the 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 wedge shaped sub-vertebral bones (c) hitherto undescribed.

   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.