SKELETON CHAP. 



present a considerable number of processes or projections, which 

 protect certain blood-vessels, and serve for the attachment of 

 the muscles which turn the flexible neck. The dorsal vertebrae 

 follow, and some not unfrequently coalesce with each other, but 

 this is always so with the sacrals, and in nearly all existing 

 Birds with the terminal portion of the caudals, which are fused 

 together to form a " pygostyle " or upright triangular plate to carry 

 the tail-feathers. 1 Archaeopteryx, so far as is known, stands 

 alone in having all the caudal vertebrae free. 



A typical vertebra consists of a centrum, and an arch, with 

 articular surfaces for two ribs, and is called heterocoelous when the 



facets, or connecting surfaces, are 

 saddle-shaped, a condition charac- 

 teristic of, and restricted to, Birds. 

 It is amphicoelous, or biconcave, 

 when each end is hollowed, as in 

 the dorsal region of Ichthyornis 



& / .^f\"^^^^J an d probably in Archaeopteryx ; 



j^ VP- procoelous, when concave in front 



FIG. l.-Third cervical vertebra of Wood- ( a8 is COmmon in ^ptiles) ; opis- 



pecker (Picus viridis). (Viewed thocoelous when concave behind 



men"of upper Lcfc ; Pa, articular ( aS * n man 7 Mammals). 



process ; Psi, haemal spine ; pt, Pt, 2. The Eibs are doubly attached 



the two bars of the transverse process, , ,-, , n ^ il j / 



shewn on one side ancyiosed with the to the vertebrae by a head (capi- 



cervical rib (R) ; Sa, articular surface tulum} and a knob (tubercullim) \ 

 of centrum. (From Wiedersheim. ) , , , , , 



and have a neck, a dorsal, and a 



ventral portion, each dorsal section (save on the last rib) 

 possessing an " uncinate process " or thin, bony posterior projec- 

 tion, except in Archaeopteryx and the Palamedeidae. Should the 

 ventral piece articulate with the sternum the rib is " true," 

 otherwise it is called " false " ; moreover the cervical and 

 frequently the post - thoracic ribs are fused with the cervical 

 vertebrae and the ilia respectively. 



3. The Breast-bone (Sternwrri) presents two different styles 

 according to whether it exhibits on its ventral surface a median 

 ridge or keel (carina), or not. In the former case, which is that 

 of by far the greater number of existing Birds (hence termed 

 Carinatae), the keel is of variable size, being correlated with the 

 power of flight. It is exceedingly deep in the Swifts, Humming 



1 The Ratitae, Crypturi and Hesperornis have no pygostyle. 



