EXPLANATION OF PLATES 12. 13. * 25 



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 Ver- 

 tebrse, 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 



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 fron- 

 tal 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 fur- 

 rowed. 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 trian- 

 gular 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. 



VOL. II. 3 



