OSTEOLOGY 115 



splints of bone lying on one side, which have been compared 

 to the ossifications in tendons found among the ptero- 

 dactyles. They may conceivably be misplaced chevron 

 bones. 



The pygostyle varies much in the degree of its develop- 

 ment. It is weakest in various aquatic birds, such as the 

 auks and grebes, where it is thin and narrow ; in more 

 purely flying birds it is very thick at the base, and is turned 

 upwards instead of, as in the auks, carrying on the line of 

 the tail. In the grebe there is really no more definite a 

 ploughshare bone than in the ostrich. The number of 

 vertebrae which are fused together to form the urostyle 

 varies. In the ostrich MARSHALL finds four, five in the 

 grebe and hornbill, six in the duck and in Eurylcemus. 



The total number of vertebrae ' in the column varies 

 greatly; the extremes are something like thirty-nine and sixty- 

 four (reckoning the urostyle as one). The greatest number 

 characterises the ratites, and the smallest some of the higher 

 arboreal birds. Archceopteryx had only about fifty vertebrae. 

 While, therefore, it may be generally true to put down as 

 older types those with the largest number of vertebrae, it is 

 evident that on this view ArchccopteryxTfm.^i be regarded as 

 a parallel branch to the existing birds, and not as their 

 ancestor. The number of vertebrae, though it may perhaps 

 be considered from this general point of view, is not of the 

 faintest use for the systematic arrangement of existing forms. 

 The number varies so extremely that among the Gruidae 

 Professor PARKER found no two alike. Rather more fixed, 

 but still subject to variation among the species of a genus, 

 are the cervical vertebrae ; and some account will be taken 

 in the pages which follow of this fixedness. The results 

 must, however, be tempered by the reflection that while the 

 common swan has twenty-five the black-necked swan has 

 twenty-four. 



Between the successive centra are the ' intervertebral 



1 GIEBEL ('Die Wirbelzahlen am Vogelskelet,' Zcitschr. f. d. ges. Nat. xviii. 

 18GG, p. 20) gives a long list ; see also ' Der letzte Schwanzwh bel des 

 Vogelskeletes,' ibid. vi. 1855. 



I 2 



