AXIAIi SKELETON OF THE STEUTHIONID^. 11 



compared with a bird's skull, has the beak smaller and the cranium less dorsally convex, 

 while the upper of the two anterior bars(?'. e. the ischium) joins the beak. by ossification 

 of the ligament uniting it with the ilium. Moreover the pelvis differs from that of 

 Stnithio in that the two bars (ischium and pubis) extend postaxiad beyond the ilium to 

 a greater degree, and do not curve ventrad distally. The supratrochanteric process is 

 also decidedly preaxiad to the antitrochanteric process, instead of slightly postaxiad as 

 in Struthio. The ilio-pectineal process is much shorter ; and the vertebral column is 

 scarcely to be seen through the acetabulum ; while the latter cavity is placed nearer the 

 middle (pre- and postaxially) of the total length of the ilium. The os innominatum is 

 much less deep dorso-ventrally in proportion to its pre- and postaxial extent. The pre- 

 acetabular part of the ilium has its external surface more concave than in Struthio, as is 

 also the preaxial margin of the ilium. 



Viewed dorsally, the pelvic ribs are relatively larger than in Struthio. The antitro- 

 chanteric processes project at about the middle of the ilium's length pre- and post- 

 axially and behind the anterior third of the total length of the pelvis. Just in front of 

 the processes the supraacetabular processes project strongly outwards, relatively as well 

 as absolutely much more so than in Struthio. In Rhea only three processes are seen to 

 extend postaxiad (instead of five as in Struthio), all joining together distally. The 

 median process of these three (ilia and ischia in one) expands distally so that its lateral 

 margins are concave. No vertebral spines are visible in the adult (as they are in 

 Struthio), the two ilia ankylosing together dorsally in the adult, and even in the im- 

 mature {e. g. in No. 1361 e) meeting together, though in the very young {e. g. in 

 No. 1361 f) the lumbo-sacral and sacral spine-tips come to the surface between the ilia 

 from a little preaxiad to a little postaxiad of the preacetabular process. The two 

 lateral processes above spoken of are, of course, the two pubes, each pubis forming a 

 gentle cui-ve (not so bowed outwardly as in Struthio) with a convex external margin and 

 a concave internal one. 



The transverse diameter of the pelvis is pretty uniform. It is greatest between the 

 ends of the pelvic ribs, and next greatest across the pubes. The posterior ends of the 

 ilia do not diverge as in Struthio. 



When viewed ventrally, the most striking differences between the pelvis of Ehea 

 and that of Struthio are, the greater size of the pelvic ribs in the former, the fact that 

 but three bony processes extend postaxiad, and the binding of the sacro-caudal vertebrae 

 and the postaxial parts of the ilia by the medianly united ischia. Also the iliopectineal 

 eminences are smaller, and the centra postaxiad to the sacral vertebrae are aborted. 



Although there are from three to five lumbo-sacral vertebrae (i. e. without parapo- 

 physes and between the lumbar and sacral vertebrae), yet they are so closely approxi- 

 mated antero-posteriorly as to leave but a very small fossa between them on each each 

 side and the adjacent acetabulum. 



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