beneath, exhibiting the course of its over lapping
bones. (Conybeare.)
A. Tooth of a Crocodile, shewing 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 shewn 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.)
P late 12. V. 1. p. 181.
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 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 Vertebra 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.
A. Hollow conical Vertebrae of a fish. (Original.)
B. C. E. Vertebrae of Ichthyosaurus. See Note, V. I.
p. 178. (Home and. Conybeare.)
D. a. g. E. a. g. Spinous processes, shewing the peculiar
articulation of their annular portions, with the Vertebrae,
to be adapted to increase the flexibility of the
spine. See Note, V. I. p. 179. (Home.)
P late 13. V. I. p. 190.
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 iti 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 hone.
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 hone.