254 Prof. H. G. Seeley on the Ormthosaurian Pelvis. 



identification of the prepubic bone, which rests on inductive 

 evidence, is the more interesting since it shows that the main 

 pelvic differences between the Ornithosauria and Ornithischia 

 are in the less breadth of the ischium, in the prolongation 

 backward of a posterior pubic process, and the greater depth 

 of the ilium. The pelvic features in which the two groups 

 agree appear to be the development of the anterior and poste- 

 rior processes to the ilium, the posterior approximation of the 

 ischium, the short development of the anterior process of the 

 pubis, and the possession of prepubic bones. The distinctive 

 pelvic character is the development of the posterior process of 

 the pubis ; and although this process attains a similar deve- 

 lopment in birds, birds have practically lost the anterior 

 processes of the pubes and have entirely lost the prepubic 

 bones. 



Thus, considering pelvic characters only, there is a certain 



Fig. IG. 



Crocodilia. 



Ornithosauria. 



Ornithischia. 



Saurisehia. Atbs. 



Diagram of Pelvic Relationship in the Ornithomorpha. 



community of structure between the Ornithosauria, Ornith- 

 ischia, and Crocodilia in the possession of prepubic bones, 

 and between Saurisehia and Aves in wanting those bones. 



