THE ANATOMY OF A BIRD 



13 



sufficiently indicates its shape ; the expanded end of the bone serves 

 as a firm base, upon which rest the strong tail feathers. Now, in the 

 ostrich tribe there are no rectrices comparable in size to those of 

 the flying carinates. Here there is no pygostyle, but the individual 

 vertebrae are small and disconnected. They are, however, few in 

 number, whereas in the Archseopteryx they are numerous, though, 

 oddly enough, not so numerous altogether as are the tail vertebrae of 

 some flying birds. Each individual vertebra in the Archaeopteryx 

 supports a pair of rectrices, which are thus arranged in a series, and 

 not in one row. A very distinctive peculiarity of the vertebrae of 

 birds is the saddle-shaped centrum. The centrum of the vertebra 

 is the solid piece which imderlies the canal of the spinal cord, the 

 walls of the latter being formed by the neural arches, which unite 

 above to form a neural spine. In other 

 vertebrates the centra are flat (mammals), 

 orprocoelous (the concavity being forward), 

 or opisthocoelous (the concavity posterior), 

 or amphicoelous (concave on both sides). 

 This latter form of vertebra is frequently 

 met with in archaic forms belonging to 

 various groups. It occurs, for example, 

 in many fishes. Such reptiles as Hypero- 

 dapedon and the Geckos have the same 

 kind of vertebrae. Among birds there is 

 no existing genus or species which is to 

 be thus characterised ; but the extinct 

 Ichthyornis had clearly biconcave ver- 

 tebrae. 



Shoulder Girdle. 



This series of bones serves as the inter- 

 mediary between the fore limb and the 

 vertebral colunm. It consists of three dis- 

 tinct elements. There is, first of all, a 

 sword-blade-like bone with sharp edges, 

 which lies along the vertebral column — 



the scapula. To the end of this is firmly attached a somev/hai 

 shorter bone, which approaches its fellow as it joins the sternum 

 below; this bone is known as the coracoid (52- fig. 8,). The angle 



Fig. 8.— Sternum of 

 Shrike. 



h, ribs ; 58, furcula ; 52, cora- 

 coid ; 59, anterior end of 

 sternum. 



