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. 178. (Home and Conybeare.) 

 D. a. g. E. a. g. Spinous processes, shewing the peculiar 

 articulation of their annular portions, with the Ver- 

 tebrae, to be adapted to increase the flexibility of the 

 spine. See Note, V. I. p. 179. (Home.) 



Plate 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 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 pres- 

 sure of the head, at the same time facilitating the rotatory move- 

 ments of the Occipital bone. 



Fig. 4. c. Second sub-vertebral wedge articulating with the trian- 

 gular cavity formed by the marginal facets of the Atlas and Axis. 

 This second Wedge acted as a strong prop supporting firmly th& 

 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 pro-- 

 jecting boss of solid bone. 



