0. C. Marsh —Vertebree of Recent Birds. 267 
In the vertebra of Jchthyornis shown in figures 1 and 2, it will 
be seen that the articulation of the centrum is cup-shaped ; 
while in the corresponding vertebra of Hesperornis, 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 Jchthyornis and any living bird. 
0 the evolutionist, who believes that birds are all closely 
connected genetically, this difference in structure, at first sight, 
offers a most serious difficulty ; since hitherto we have bad no 
hint of a transformation from the one form to the other, and no 
explanation of the origin of the modern vertebre of birds. 
L. 2. 3. 
Figure 1.—Twelfth (?) cervical vertebra of Ichthyornis dispar, Marsh; front view; 
twice natural size. 
FiguRE 2.—The same vertebra; seen from the left side. : 
Figure 3.—Third cervical vertebra of Ichthyornis victor, Marsh; front view; twice 
natural size. 
Figure 4.—Thirteenth cervical vertebra of Hesperornis regalis, Marsh ; front view ; 
natural size, 
GURE 5._-The same vertebra; posterior view. _ : 
a. anterior articulation ; d, diapophysis; p. parapophysis ; o lateral 
foramen; ne. neural canal; s. neural spine; 2 pre-zygapophysis; 2’. post- 
zygapophysis. 
In the third cervical vertebra of Jchthyornis, however, we 
catch nature in the act, as it were, of forming a new type; 
