PHYLOGENETIC 



33 



the adult they coalesce with the centrum, while in the nature of 

 the ends of the body, or centrum, of the vertebra, reptilian 

 blood is also evident. But this evidence might not, at first, 

 appear so plainly as in the 

 characters just referred to, since 

 the majority of birds have now 

 acquired what are known as 

 " saddle-shaped " vertebrae, or 

 heterocoelous vertebrae ; that is 

 to say, the two ends are unlike. 

 Anteriorly the articular surface 

 is columnar, posteriorly trans- 

 versely hollowed, so that the 

 columnar surface of the vertebra 

 moves, laterally, on the hol- 

 lowed surface of that next in 

 front of it. But in the old Ich- 

 thyornis like the modern reptile 

 Hatteria, these surfaces were 

 cup-shaped ; while the Par- 

 rots, Penguins and Gulls have 

 cup and ball articulations to the 

 vertebrae, in the region of the 

 thorax, and these are quite rep- 

 tilian in type. Young birds, 

 however, furnish much more 

 emphatic ' testimony on this 

 point, for, in addition to the 

 movable ribs of the neck verte- 

 brae, we have the fact that, as in 

 reptiles, these vertebrae do not 

 possess, at any stage, " Epi- 

 physes," as in the mammals. 

 These epiphyses are flat discs of 

 bone fitting on to the ends of 

 the vertebrae, enabling new bony 

 matter to be added between the 

 enclosed space : growth com- 

 pleted, these plates fuse with the main body of the centrum. 

 The hip-girdle, especially during its embryonic stages, pre- 

 3 



III. g. — Hip-girdles of a Dinosaur 

 (A), AN Embryo Bird (B), and a 

 Nestling Emu (C) 



The hip-girdle of the embryo in 

 the shape of the Ilium (II.) and the 

 position of the Pubis (P.) and Ischium 

 (Is.) resembles that of the Dinosaur — 

 a primitive or ancestral phase of de- 

 velopment. In the nestling Emu the 

 Ilium has greatly increased in length, 

 and the Pubis and Ischium have 

 rotated backwards. In less primitive 

 birds the fissure betvifeen the Ilium 

 and Ischium becomes almost com- 

 pletely closed. 



