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 preesacrals, and especially of those 
of the ischiadie portion (2). 
So far then the general plan of the Sacrum is easily understood, 
but since it is composed of numerous vertebre 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 formule. The few illustrations 
here introduced will serve to indicate some of the differences. 
The Caudal Vertebre 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, which has slightly heteroccelous 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), Scapulee 
and CLAVICLES (page 89), which last three meet and form the fora- 
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 
zur Morphologie 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 Preecora- 
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 Preecoracoid—a bone which is almost 
typically complete, although soon fused at either end with the 
