484 MANUAL OF ZOOLOGY. 



the last dorsal and the first caudal (varying from nine to 

 twenty) are anchylosed together to form a bone which is ordi- 

 narily known as the "sacrum." To this, in 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 vertebrae vary in number from 

 eight to ten, and are movable upon one another. 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, 'stemfef, ploughshare-shaped bone, 

 destitute of lateral processes, and without any medullary canal 

 (fig. 214, B). In reality it consists of two or more of the 

 caudal vertebras, 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 perpendicular to the axis of the 

 body ; and it affords a firm basis for the support of the great 

 quill-feathers of the tail ("rectrices"). It also supports the 

 coccygeal oil-glands, and can be raised at pleasure, so as to 

 meet the bill, when the operation of preening is in progress. 

 In the Cursorial birds, which do not fly, the terminal joint 

 of the tail is not ploughshare-shaped. In the extraordinary 

 Mesozoic bird, the Afck&opteryx macrura, there is no plough- 

 share 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 (fig. 232). As the vertebrae of the 

 ploughshare-bone are distinct from one another in the em- 

 bryos of existing birds, the tail of the Archaopteryx 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 Archceopteryx makes a decided approach to the true 

 Reptiles. 



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 remaining 

 movable. The occipital bone carries a single occipital condyle 

 only, and this is hemispherical or nearly globular in shape. The 

 "beak" (fig. 211), which forms such a conspicuous feature in 

 all birds, consists of an upper and lower half, or a " superior " 

 and " inferior mandible." The upper mandible is composed 

 almost entirely of the greatly elongated intermaxillary bones 

 flanked by the comparatively small superior maxillae. The 

 inferior mandible is primitively composed of twelve pieces, six 

 on each side ; but in the adult these are all indistinguishably 

 amalgamated with one another, and the lower jaw forms a single 



