VERTEBRATA : AVES. 575 



highly characteristic of Birds. The dorsal vertebras vary from 

 six to ten in number, and of these the anterior four or five are 

 generally anchylosed with one another, so as to give a base of 

 resistance to the wings. In the Cursorial Birds, however (such 

 as the Ostrich and Emeu), and in some others (such as the 

 Penguin), in which the power of flight is wanting, the dorsal 

 vertebrae are all more or less freely movable one upon another. 

 There are no lumbar vertebras, but all the vertebras between 

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

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

 dinarily 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 vertebras vary in number from eight 

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

 ever, the number of caudal vertebras is much greater than the 

 above, since some of the vertebras of the anchylosed " sacrum " 

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

 share-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, ploughshare-shaped bone, 

 destitute of lateral processes, and without any medullary canal 

 (fig. 325, 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-gland, 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 Arch&opteryx macrura, there is no plough- 

 share-bone, and the tail consists of twenty separate vertebras, 

 all distinct from one another, and each carrying a pair of quill- 

 feathers, one on each side (fig. 351). As the vertebras of the 

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

 bryos 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 vertebras, 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 



