AXIAL SKELETOiS' OF THE PELECAJSTTD^. 327 



postaxial end of the centrum than in the seventeenth vertebra. There are similar 

 par- and diapophysial surfaces, but further apart. 



Defects of ossification, which began to appear in the preceding vertebra between 

 these surfaces, on the outsides of the transverse processes, are here very conspicuous ; and 

 sometimes a large oval foramen (placed between the postzygapophysis and the adjacent 

 postaxial margin of the transverse process) leads into the substance of the haemal arch 

 on each side. 



The NINETEENTH VEKTEBKA is the first vertebra which forms, in the adult, part of the 

 sacral mass, being more or less ankylosed to the vertebra next succeeding. It is 

 slightly smaller, in all dimensions, than is the eighteenth vertebra; but the lateral 

 defect of ossification is much larger and is the conspicuous lateral opening, the post- 

 axial intervertebral foramen being much smaller than that between the eighteenth and 

 nineteenth vertebrae. 



The TWENTIETH and twenty-first vertebra repeat the characters of the nineteenth, 

 except that they are slightly smaller and more and more involved in the preaxiad 

 extension of the iliac ossification, which completely covers the dorsum of the twenty- 

 first vertebra, except that a foramen opens (dorsally to the transverse process), ex- 

 tending obliquely preaxiad and dorsad. 



The rib-surfaces are rather less far apart in the twenty-first than in the twentieth 

 vertebra ; in the latter they are at their maximum of separation. 



The twenty-second vertebra is still smaller, and is overlapped by the ilium itself. 

 The di- and parapophysial surfaces are much more approximated. The defects of 

 ossification are less extensive, though there is much individual variation in this respect. 



The dorsal end of the transverse process sends out postaxially an arched ossification 

 (concave ventrally) which meets a similar but preaxially extending arched process from 

 the preaxial end of the dorsal part of the transverse process of the twenty-third vertebra. 

 Thus, when these vertebrae are viewed laterally, there is the appearance of a bony arch 

 extending up dorsally between and connecting them (Plate LIX. figs. 1 & 3, xsii). 



The Lumbar Vertebra. 



In comparing these vertebrae with the so-called lumbar vertebrae of Strut/do, and 

 with the corresponding vertebrae of most of the other Struthionidae, a striking difference 

 is apparent. It consists in the marked differentiation which exists in Pelecanus between 

 those more preaxial lumbar vertebrae which send out parapophysial transverse pro- 

 cesses, and those more postaxial ones which only develop ascending diapophysial 

 processes, and thus leave a wide interval between their vertebral centre and the aceta- 

 bula — i. e. a lateral acetabular or renal fossa on each side, as will be more fully noticed 

 in describing the ventral aspect of the pelvis. 



These vertebrae, therefore, are more evidently divisible into true lumbar vertebra and 



