AXIAL SKELETON OF THE STEUTHIONID.E. 3 



processes of which abut against the ilia just behind the acetabula (fig. 8, xxxiii, d ikp, 

 and fig. 9,^;). After these vertebras there is a sort of gap, the nine anterior caudal 

 vertebrae either becoming entirely absorbed between the ilia or never being distinctly 

 developed. Five imperfect, half-absorbed vertebrae appear postaxiad of the ilia ; and to 

 them succeed six free and, in a sense, less incomplete vertebrae, which terminate the 

 series. Thus there are but twenty-three vertebrae belonging to the first four categories, 

 instead of twenty-seven as in Struthio. 



THE CEEVICAL VERTEBRA. 



ATLAS AND AXIS OF RHEA (natural size). 

 Fig. 3. 



Fig. 3. Preasial view of atlas. Fig. 4. Lateral view of axis. 



ac.prearial articular surface of centrum; (Z, diapophysis ; Ay, median hypapopliysis ; M, lateral hj-papophysis ; aj, prsejygapo- 

 physis ; hp, hyperapophyais ; ms, neural spine ; o, odontoid process ; pc, postaxial articular surface of centrum ; pi, pleural 

 lamella ; p^, postzygapophysis of axis ; z, postzygapophysis of atlas. 



The atlas, compared vrith that of Struthio, is relatively much more dorso-ventrally 

 and less transversally extended. The cup for the occipital condyle is nearly complete, 

 having but a very small and narrow dorsal excavation. The lateral hypapophysial pro- 

 cesses (fig. 3, Ih) are more marked, and the median hypapophysis relatively smaller. The 

 diapophyses (fig. 3, d), though distinct, are very much, even proportionally, smaller. No 

 hyperapophyses are distinguishable ; nor is there any costal bony spiculum. Even in 

 the very immature specimen in the Museum of the College of Surgeons (No. 1361 f). 

 the neural laminae are entirely ankylosed to the quasi-body, and show but a trace of 

 their own dorsal union. 



The axis (fig. 4) has a greater vertical extent than that of Struthio, while its antero- 

 posterior excess over the atlas is much less. There is a pit below the odontoid process. 

 The postaxial central surface is much more prolonged ventrad by the relatively much 

 larger hypapophysis (fig. 4, 7w/), which also extends further towards the preaxial margin 

 of the centrum. The neural spine (fig. 4, n s) is relatively shorter (pre- and postaxially) 

 and higher. The postzygapophyses are still more in excess of the praezygapophyses than 

 in Struthio. A very large pneumatic foramen is placed above the parapophysial root 



b2 



