THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



[SIXTH SERIES.] 

 No. 39. MARCH 1891. 



XXV. — The Ornithosaurian Pelvis. By H. G. Seeley, 

 F.R.S., Professor of Geography in King's College, London. 



In vertebrate osteology the arrangement of the bones which 

 compose the pelvic girdle contributes one of the most dis- 

 tinctive characteristics of a natural group of animals. It may 

 pervade a subclass or be limited to an order. It constitutes a 

 plan of structures which never varies so far from its type as 

 to merge in the pelvic plan of another animal type. It thus 

 becomes a convenience in classification. And when the pelvic 

 structures of different groups of animals are compared and 

 arranged in order of their community of plan, they constitute 

 a classification which is often suggestive of original commu- 

 nity of structure. The existing warm-blood groups of 

 animals — Monodelphia, Didelphia, Ornithodelphia, Aves — 

 are remarkable for fixity of pelvic plan ; but it is among Aves 

 that variation has the widest range, so that median symphysis 

 of the pelvic bones in the genus Struthw, for example, makes 

 a divergence from the Ratite pelvic plan, which shows that 

 no importance in classification necessarily attaches to such a 

 condition of ossification. The more variable pelvic plans of 

 the Reptilia and Amphibia suggest that the ordinal groups in 

 those classes of animals have the morphological value of sub- 

 classes when compared with the orders of Birds or Mammals. 

 Ann. & Mag. N. Hist. Ser. 6. Vol. vii. 17 



