256 Transactions, — Zoology. 



a group must have been intermediate between Odontornithes^' and Carinatm 

 is quite sufficient to account for all the reptilian affinities of the Ratitm : 

 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 Ratitce make no approach whatever to reptiles. The 

 same hypothesis also explains the fact that while the Ratitm 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 

 differ in the most remarkable way in other points. For instance, while 

 Dinornis retains such 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 the Ratit(B 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 Dldus. The Proto-Carinatm 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 Katite plumage should be looked upon as a 

 secondary or degenerate, not as an ancestral, character. 



Finally, if we look upon the typical Odontornithes and Archaopteryx as 

 approximately linear types in the ancestry of birds, we must assume that 

 the latter arose from ornithoscelidan reptiles of comparatively small size, 

 the gigantic Dinoscmria 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 : — 



Carinats 



-Proto-Carinatffl 



Batitffi 



Odontornithes 

 Dinosauria 



Saururffi 



* I mean such Odontornithes as Ichthyornis : Hesperornis is another instance of 

 degeneration induced by disuse of the wings. 



