24 
EXPLANATION OE I’LATES 11. 12. 
beneath, exhibiting the course of its over lapping 
bones. (Conybeare.) 
A. Tooth of a Crocodile, shewing the incipient absorp- 
tion 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.) 
Plate 12. V. 1. p. 181. 
1. Sternal Arch and Paddles of Ichthyosaurus. See V. 1. 
p. 182, Note. (Home.) 
2. Sternal Arch of Ornithorhynchus. (Home.) 
3. 4, 5, 6. Occipital and Cervical Bones of Ichthyosau- 
rus, from the Lias at Lyme Regis.* (Original.) 
* Sir Philip de Malpas Grey F.gortoii has pointed out some beau- 
tiful examples, hitherto unnoticed, in the Atlas and cervical Ver- 
tebrae of Ichthyosauri, of peculiar mechanical contrivances to sup- 
port and regulate the movements of their enormous heads. (See 
Lond. and Ediii. Phil. Mag. Nov. 183o. 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 sur- 
faces, admitting of the least flexure of any of the Vertebrm in the 
whole body, but giving the greatest strengtlj to that part of the 
Column, where strength rather than flexure was retpiired. 
On the inferior margins of the 4tlas and Axis and third cervical 
vertebra, are triangular facets articulating with three strong wedge-- 
shaped suh-vertebral bones (c) hitherto nndescribed. 
