OSTEOLOGY 119 



nearer and nearer, until at length they mount upon the 

 hsemapophysis itself and pass to its very end. Details of the 

 formation of the vertebrae vs^ill be found in the systematic 

 part of this book.' 



Ribs.^ — The ribs of birds vary greatly in number. There 

 are as a rule three series of ribs to be distinguished. The 

 last cervical vertebrae, more or fewer of them, are furnished 

 with short ribs which do not reach the sternum. Behind 

 these are, again, a variable number of true ribs, which do 

 reach and articulate with the sternum. These true ribs 

 consist of the vertebral portion, which articulates with the 

 vertebra, and of a sternal portion, which is articulated vnth 

 the vertebral half of the rib above and with the sternum 

 below; it is bent at an angle with the vertebral portion. 

 Attached to, originally separate from, and sometimes per- 

 manently separate from, the vertebral half of the rib is the 

 uncinate process, of which there are a variable number. 

 These processes are absent^ in Archaopteryx and in the 

 Falamedese only. Behind the true ribs, which articulate 

 with the sternum, are a variable number, in all degrees of 



' The relationship of the so-called eatapophyses to the unpaired hsemapO- 

 physes varies, and suggests — what has been advanced on other grounds — aft 

 occasional escalation of vertebrae. Without wishing to eommit myself to a 

 belief in the actual dropping out of a vertebra •from the middle of the series, I 

 may mention some of the facts which may be regarded as pointing in this 

 direction. In the grebe ^chmophorus the eatapophyses form on certain 

 vertebrsB a complete ventral canal for the carotids. The summit of the arch 

 thus formed gradually acquires a median dorsal process. This increases, and 

 the eatapophyses finally end in the obliteration of the canal which they sur 

 round, and a solid arch is formed ; the hypapophysis of the succeeding vertebra 

 is single and no longer retains traces of its evolution from a ring of bone sur- 

 mounted by a process. In other cases the eatapophyses suddenly end and the 

 hypapophyses begin without such intermediate stages. An intermediate stage 

 is seen in certain types where the eatapophyses end suddenly, but the first 

 hypapophysis is double, either formed of two clearly fused pieces or with merely 

 a bifid spine. These latter cases suggest the dropping out of one or more 

 vertebras, effecting the transition between the paired eatapophyses and the un- 

 paired hypapophyses. 



' In all birds except Archaopteryx the ribs are two-headed with a capitulum 

 and tuberculum. 



" They have been often said to be absent in Dinornis, but they are not. 



W. Behkens, Untersuchungen Uber den Processus imcinatus der VOgel uncL 

 Crocodile, Inaug. Diss., Gottingen, 1880. 



