244 BIRDS. 



turn, the iliac bones are anchylosed along their whole 

 length, giving perfect immobility to this region of the 

 spine and to the pelvis. 



The coccygeal or caudal vertebrse vary in number from 

 eight to ten, and are movable upon one another. In reality, 

 however, the number of caudal vertebrse is much greater 

 than the above, since some of the vertebra3 of the anchylosed 

 " sacrum " properly fall to be counted in this region, and the 

 " ploughshare -bone " consists of more than one vertebra. 

 The most noticeable feature about this part of the spinal 

 column is what is known as the " ploughshare-bone." This 

 is the last joint of the tail, and is a long, slender, plough- 

 share-shaped bone, destitute of lateral processes, and without 

 any medullary canal (fig. 584, b). In reality it consists ot 

 two or more of the caudal vertebrae, completely anchylosed, 

 and fused into a single mass. It is usually set on to the 

 extremity of the spine at an angle more or less nearly per- 

 pendicular to the axis of the body ; and it affords a firm 

 basis for the support of the great quill-feathers of the tail 

 (" rectrices "). In the Cursorial Birds, which do not fly, the 

 terminal joint of the tail is not ploughshare-shaped. In the 

 extraordinary Mesozoic bird, the ArclimopUryx macrura, there 

 is no ploughshare -bone, and the tail consists of twenty 

 separate vertebrae, all distinct from one another, and each 

 carrying a pair of quill-feathers, one on each side. As the 

 vertebrae of the ploughshare -bone are distinct from one 

 another in the embryos of existing birds, the tail of the 

 Archmopteryx is to be regarded as a case of the permanent 

 retention in the adult of an embryonic character. In the 

 increased number of caudal vertebrae, however, and in some 

 other characters, the tail of the Archceoptcryx makes a de- 

 cided approach to that of the true Eeptiles. 



The various bones which compose the skull of Birds are 

 amalgamated in the adult so as to form a single piece, and 

 the sutures even are obliterated, the lower jaw alone remain- 

 ing movable. The occipital bone carries a single occipital 

 condyle only, and this is hemispherical or nearly globular in 

 shape. The "beak" (fig. 581), which forms such a conspic- 

 uous feature in all birds, consists of an ujjper and lower half, 



