1902.] OSTEOLOGY OP THE FALCONIFORMES. 295 



able from the postsacral (caudal) vertebrae by reason of the long 

 slendei' venti'o-lateral pi-ocesses of these last, which in no respect 

 differ in appearance from the sacral libs. Thus the posterior 

 renal fossa {fossa 2>udendcdis), which is large, is cut up into a 

 number of sepai'ate compartments. 



There is a certain resemblance between the synsacra of Pandion 

 and Cathartes. But the two may readily be distinguished. In 

 Pa7idion the lateral iliac fossa (fossa lumharis) is peculiar on 

 account of the great size of the ventri- lateral processes, the outer 

 ends of which are fused one to another to form a bony bar, upon 

 which the preacetabular ilium rests. The dorsi-lateral processes 

 forming the roof of the anterior and posterior renal fossae give 

 rise to a large, flat, imperforate plate with convex free borders, 

 showing a distinct suture-line betwixt them and the postace- 

 tabular ilia. The intervertebral foramina are almost completely 

 obliterated. 



In Cathartes the ventii-lateral processes of the lateral iliac fossa 

 are less conspicuously developed. The roof of the anterior and 

 posterior renal fossae is perforated by large intervertebral foiumina ; 

 and the lateral edges of the plate forming the roof of this fossa 

 is deeply hollowed both before and behind the sacral vertebrae. 

 There are 6 to 8 postsynsacral vertebrae (free caudals) including 

 the pygostyle. The anterior of these vertebrae bear vestiges of 

 the anterior zygapophyses, which gradually decrease in size fi'om 

 before backwards. 



In Serpentarius the neural spines are swollen and bifid, the 

 transverse processes are pierced at the base by comparatively 

 large pneumatic apertures. The last 3 or 4 vertebrae bear bifid 

 hypapophyses. The free caudals of Gypaet%i,s resemble those of 

 Serpentarms but are non-pneumatic. In the larger Eagles the 

 transverse processes are mu.ch expanded and very wide. The 

 number of caudal vertebrae included in the synsacrum appears to 

 vary between 1 {8pizaet%i.s^^rL^^{fierpentarms). In the Falconidae 

 the pygostyle bears a pair of accessoiy plate-like vesicles, attached 

 by their hinder border by ligament to its inferior angle, and pro- 

 jecting forwards as a hoi-izontal plate. Intercentra occui-. 



iv. The Ribs. 



Tlie cervical ribs extend from the third vertebra backwards, 

 and are ankylosed with theii" respective vertebrae. They are 

 relatively longest in Serpentarius. The free, so-called cervico- 

 thoracic belong more correctly to the thoracic series. 



The thoracic ribs range from 5 [Pseudogyps, Polyhoroides) to 8 

 pairs. The 8th pair, however, is never complete : often, as in 

 Sp)izaetus for instance, only the stei-nal segment is present, fixed 

 by membrane to the 7th pair of sternal ribs. Sometimes the 

 thoracic and stei-nal segments are widely separated by loss of the 

 intermediate ossified tissue, as in a case of Aqtiila chrysaetos. 

 "Whilst in other cases the thoracic and sternal segments are 



