256 Transactions.—Zoology. 
a group must have been intermediate between Odontornithes* and Carinate 
is quite sufficient to account for all the reptilian affinities of the Ratite: 
the assumption that they had acquired the distinctive characteristics 
of shoulder-girdle and sternum, pelvis, vertebral column, and fore and 
hind limbs, and had lost all trace of teeth, will account for the fact that 
in these points the Ratite make no approach whatever to reptiles. The 
same hypothesis also explains the fact that while the Ratite agree in certain 
common characters, some because they are of ancestral importance, others 
because they have been acquired by the same law of degeneration, they 
alee in te most remarkable way in other points. For instance, while 
h ancestral structures as the hallux, large external xiphoid 
processes, and free ischium and pubis, it has undergone the greatest amount 
of degeneration of the shoulder-girdle and forelimb. Struthio, in the same 
way, shows the extreme of modification in the foot, Struthio and Rhea in the 
pelvis, and so on. 
Probably the great size of thus Ratite is also connected with their cursorial 
life, and is, like so many points in the skeleton, merely an exaggeration of 
what is found in Notornis, Cnemiornis and Didus. The Proto-Carinate being, 
by the hypothesis, good fliers, were presumably not of gigantic size ; more- 
over they probably possessed feathers with connected barbs, so that the 
special characters of the Ratite plumage should be looked upon as % 
secondary or degenerate, not as an ancestral, character. 
Finally, if we look upon the typical Odontornithes and Archaoptery= 98 
approximately linear types in the ancestry of birds, we must assume that 
the latter arose from ornithoscelidan reptiles of comparatively small size, 
the gigantic Dinosauria being a special development of the same type- 
The following diagram expresses the results to which a consideration of 
the above facts seems to lead :— 
Carinatz2 
* I mean gs such Odontornithes as Ichthyornis : Hesperornis is another i 
degeneration induced by disuse of the wings. 
