VERTEBRAL COLUMN. 45 



the sacrum even a greater number may be included (cp. the chapter 

 on the pelvis, p. 96) : thus in the common Duck (Anas boschas), 

 seven become united with the pelvis, eight remain free, and the 

 pygostyle is composed of ten separately ossified and fused segments, 

 making in all twenty-five vertebrae originally present in the caudal 

 region of this Bird. In Archaeopteryx the pelvis was much shorter 

 than in existing Birds, and much fewer vertebras were united with 

 it. Moreover, in embryos of an Australian parrot (Psittacus undu- 

 latus) more vertebrae are formed in the embryo than are seen in the 

 adult. The original type is well preserved to the present day in 



FIG. 34. PELVIS OF OWL (Strixbubo). (Ventral view.) 



W, position of the primary sacral vertebrae : between R and II, and behind W, are 

 seen the secondary sacral vertebrae, fused with the primary ; II, ilium ; Is, 

 ischium ; P, pubis ; t, foramen between ilium and pubis ; R, last pair 

 of ribs. 



the Ratitse, in which the posterior caudal vertebrae remain free, 

 instead of uniting to form a pygostyle, and the secondary sacral 

 vertebras remain longer distinct. Thus the chasm between 

 Archaeopteryx and existing Birds is in this respect essentially 

 lessened. 



The arches always become united into a single mass with the 

 corresponding centra, and are no longer separated from them for life 

 by sutures, as seen e.g. in Crocodiles, and exceptionally in Chelo- 

 nians. The same may be said of the atlas and axis, in which also no 

 sutures persist between the different parts. In the cervical region, 

 where by means of saddle-shaped articulations the vertebrae are 

 able to move easily on one another, the bifurcated transverse 



