856 SKELETON 



sacral not unfrequently retains a pair of rib-elements which either 

 abort or form a third primary sacral vertebra, while on the other 

 hand only one primary sacral may exist. The general tendency of 

 modern Birds seems to be towards an increase in the number of 

 postsacrals at the expense of the prsesacrals, and especially of those 

 of the ischiadic portion (2). 



So far then the general plan of the Sacrum is easily understood, 

 but since it is composed of numerous vertebrae and those of each of 

 its constituent portions are variable in number, beside shewing 

 many modifications in the development, fusion or suppression of 

 their processes, it follows that the whole Sacrum of not only every 

 Family but even genus and almost species of Bird may have its 

 own characteristic points. These however are difficult to describe, 

 and their morphological meaning is still more difficult to recognize. 

 Thus this part of the Skeleton has hitherto escaped the pursuit 

 of the claptrap hunter of taxonomic formulae. The few illustrations 

 here introduced will serve to indicate some of the differences. 



The Caudal Vertehrse have strong transverse processes, and the 

 spinous process often shews a slight bifurcation at the end. Their 

 hypapophyses, whether double or single, are mostly restricted to 

 the last which are free and to the first of those which fuse to form 

 the PYGOSTYLE (page 753). They articulate almost entirely by the 

 centrum, Avhich has slightly heterocoelous or concave facets, with the 

 interposition of a fibrocartilaginous disk, the ventral side of which 

 frequently displays in embryos, but rarely in the adult, a median 

 osseous nodule, the last remnant of the basiventral elements com- 

 monly called the intercentrum. 



The Pectoral Arch, or Shoulder-Girdle as some term it, con- 

 sists of the Sternum and a pair of CORACOIDS (page 104), Scapulae 

 and Clavicles (page 89), which last three meet and form the /oro- 

 men triosseum, through which passes the tendon of the m. supracora- 

 coideus (pages 605, 606) to the tuberculum superius of the Humerus 

 (pages 439, 440). The configuration of the various processes of these 

 bones is manifold, and of great taxonomic importance, as has been 

 exhaustively shewn by Prof. Fiirbringer, in whose Untersuchungen, 

 znr Morphologic und Systematik der Vogel about one hundred figures 

 of this articulation in different Birds are given. 



The Coracoid is one of the most characteristic bones of the 

 ornithic Skeleton. At its upper end is the Acrocoracoidal pro- 

 cess, on the inner surface of which the proximal portion of the 

 clavicle nearly always rests ; but more important is the Praecora- 

 coidal process, of variable size and shape, arising from the inner 

 surface of the " neck " of the bone, and the remnant of an originally 

 independent element, the Praecoracoid — a bone which is almost 

 typically complete, although soon fused at either end with the 



