Geology and Mineralogy considered with reference to Natural Theology/Plate 12

by William Buckland

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.)

A. Hollow conical Vertebrae of a fish. (Original.)

B. C. E. Vertebrse 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 Vertebræ, 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 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.)

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.