CHARACTERS OF AVES. 457 



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 immo- 

 bility to this region of the spine and to the pelvis. 



The coccygeal or caudat 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, slender, ploughshare-shaped bone, de- 

 stitute of lateral processes, and without any medullary canal 

 (fig. 182, B). In reality it consists of two or more of the 

 caudal vertebras, completely anchylosed, and fused into a 

 single iTiass. 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 pro- 

 gress. In the Cursorial Birds, which do not fly, the terminal 

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

 nary Mesozoic bird, the Archaopteryx macrura, there is no 

 ploughshare-bone, and the tail consists of twenty separate 

 vertebra, all distinct from one another, and each carrying 

 a pair of quill-feathers, one on each side (fig. 20B). As the 

 vertebrae of the ploughshare-bone are distinct from one another 

 in the embryos of existing birds, the tail of the Archceopteryx 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. 179), which forms such a conspicuous feature in all 

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

 " inferjor mandible." The upper mandible is composed almost 

 entirely of the greatly elongated intermaxillary bones, flanked 

 by the comparatively small superior maxillas. The inferior 

 mandible is primitively composed of twelve pieces, six on each 

 side; but in the adult these are all indistinguishably amal- 

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



