Cervical Vertebr(x of the Ichthyosaurus, 191 



furnished with three subvertebral bones, is characterized by an articulating- 

 facet on its anterior margin. (Plate XIV. fig. 11. a.) Certain modifications 

 appear also to have obtained in the form of the intervertebral cavities in this re- 

 gion of the spine, producing a diminution of capacity to a greater or less extent 

 as compared with those more distant from the head. A remarkable instance of 

 the contraction of the cavity, combined with other striking peculiarities is 

 shown at figs. 9 & 10. The vertebra there represented is seven inches 

 in diameter, and was probably the fourth. It belonged to the same animal 

 as the large atlas already mentioned. The anterior cavity is here reduced to 

 a small cylindrical depression, not larger than a shilling in circumference, 

 (fig, 9. a.) The remaining surface is nearly flat for articulation with 

 the antecedent vertebra. The fifth vertebra resembles the fourth, but 

 is somewhat smaller. Both are furnished behind with conical cavities of no 

 great depth, for the reception of intervertebral matter. These bones exhibit 

 a remarkable contrivance, by which the security of their articulation was 

 much increased, although at the expense of a small amount of flexibility. In 

 front of each, near the lower margin, a depression is so situated as to receive 

 a corresponding eminence projecting from the contiguous vertebra 

 (figs. 9, 10. 6.), whilst on the upper margins similar depressions and projec- 

 tions are placed in the inverted order, affording twofold checks against the 

 danger of dislocation. (Figs. 9, 10. c.) 



The combined result of this construction, and of the reduction of the in- 

 tervertebral cavities, must have been a considerable increase of power in this 

 part of the spinal column. I shall only further remark that, proceeding from 

 the lumbar vertebras towards the head, the column attains its minimum dia- 

 meter about the fifth cervical vertebra, from which point to the occiput it in- 

 creases in size very rapidly. 



Having concluded the anatomical details, I now proceed briefly to examine 

 how far the structure I have endeavoured to describe agrees with the design 

 of the other parts of the animal fabric, in adapting the Ichthyosaurus to the 

 peculiar conditions under which it was destined to exist. 



Whether we consider the osteoloffical characters which I have detailed indi- 

 vidually, or collectively, in their most contracted, or in their most extended state 

 of development, we see in them provisions well calculated to increase, to a 

 greater or less amount the power of the cervical region. We also see that this 

 increaseof power, is combined withonly a slightdecreaseof the flexibility, so ad- 

 mirably provided for in the other parts of the spinal column. Letus now inquire, 

 how far this combination of motion and strength in the neck of the Ichthyo- 



