ADAPTATIONS OF THE SKELETON 



67 



In the extinct Cretaceous Ichthyornis the vertebrae were 

 amphicoelous (hollow at both ends, like a double egg-cup), 

 which is the shape characteristic of fishes. It occurs, 

 however, in many ex- 

 tinct reptiles and in the 

 extant geckos and the 

 New Zealand "lizard," 

 Sphenodon. It is rather 

 interesting that the pro- 

 coelous type of vertebra 

 (hollow in front), which 

 is commonest in modern 

 reptiles, never occurs in 

 birds. 



Several adaptations of 

 the bird's backbone have 

 been already noticed — 

 the fusion of thoracic 

 vertebrae in flying birds, 

 to form a solid basis 

 against which the wings 

 can work ; the complex 

 coalesced syn-sacrum to 

 which the ilia are fused 

 over a long stretch, to 

 facilitate the balancing 

 of the body in bipedal p^^ , 

 progression ; and the 

 terminal fusion of caudal 

 vertebras in a plough- 

 share which serves as a 

 base for the tail feathers. 

 It remains to notice 

 that the neck is usually 

 very long, with a very variable number (8-24) of very 

 mobile vertebrae, whereas mammals, with four exceptions, 

 have seven. The neck of the bird must be long 

 enough to reach the ground and to reach the preen-gland 



Dorsal vertebra of an ostrich. 

 The upper one shows the anterior 

 surface ; the lower one the posterior 

 surface. From a specimen. n.sp., 

 neural spine ; t.p., transverse pro- 

 cess ; A,z., anterior articular process ; 

 N.A., neural arch, in shelter of which 

 the spinal cord lies ; c, the centrum or 

 body ; p.z., posterior articular process ; 

 R., facet for the head of a rib. 



