180 



ODOXTORxrnn.s. 



Not only do the presacral vertebrae of all existing birds exhibit this 

 structure, but the many extinct species now known from the whole series of 

 Tertiary deposits have the same articulation. If we knew only these fossil 

 forms, in addition to the existing species of birds, the origin of this peculiar 

 vertebral articulation would perhaps remain a mystery. Most fortunately, 

 however, one of the vertebrae of Ichthyomis throws much light on this 

 point, and virtually explains the difficulty. 



We have seen that Hesperomis has the same kind of vertebrae as 

 modem birds, while those of Ichthyomis are biconcave. The marked 

 contrast between the shape of the vertebral articulation in these two 

 genera is seen in the figures below, which show a characteristic cervical 

 vertebra in each form. In the vertebra of Ichthyomis shown in figures 32 

 and 33, it will be seen that the articulation of the centrum is cup-shaped ; 

 while, in the corresponding vertebra of Hesperomis, the ends of the centrum 

 are saddle-shaped, as in ordinary birds. Thus the distinction between the 

 two types in this part of the skeleton is as wide as between Ichthyomis and 

 any living bird. 



Figure 32. — Twelfth cervical vertebra of Ichthyomis dispar, Marsh; front view; twice natural size. 



Figure 33. — The same vertebra; seen from the left side. 



Figure 34. — Third cervical vertebra of Ichthyomis victor, Marsh ; front view ; twice natural size. 



Figure 35. — Thirteenth cervical vertebra of Hesperomis regalis, Marsh; front view; natural size. 

 Figure 36. — The same vertebra; posterior view. a. anterior articulation; d. diapophysis; p. parapophy 

 J. lateral foramen; nc, neural canal: s. neural spine ; z. pre-zygapophysis; z'. post-zygapophysis. 



