1888.] 



On the Vertebral Chain of Birds, 



477 



and segmented centrum of the atlas, whilst the much larger bony 

 centre to which it is attached, and which also is strung upon the 

 notochord, is the centrum of the axis ; they coalesce together, accord- 

 ing to the rule, a new rule and part of the general transformation of 

 an Amniotic type. But the so-called body of the atlas is in position 

 between as well as below the occipital articulation, and is cortical. 

 The lesser and foremost bone in the axis is also intermediate between 

 as well as below the true centrum of the atlas and of the axis. I 

 quite agree with Baur that these two bones are intercentra, although 

 I am not ready with the " strong reasons " he can bring from every 

 corner of Palaeontology. 



In considering both intercentra and ribs, there are two birds that 

 have helped me most; these are the Swan and the Cormorant (Phala- 

 crocorax carbo). 



Whether faster or more slowly, the transformation of these two 

 types from a Reptilian into an Avian form is certainly well worthy 

 of our closest attention. The Ostrich tribe, a sort of half-way 

 creatures, only help a little in this research ; yet in tracing the stages 

 of a Swan or of any other of the Anatidae, there would appear 

 to be nothing strange in the sudden arrest of one at the Struthious 

 stage ; we seem for a time to have before us a new kind of short- 

 legged and web-footed bird of the Ostrich kind; it does move, 

 however, it develops into a Carinate bird with a Desmognathous 

 palate. I lay stress upon this, because, as I shall soon show, the 

 Anatidae hold with the Ratitae in the matter of a perfect series of 

 cervical ribs, as in the Crocodile, but more aborted, and soon fused 

 with the vertebrae. Birds are very uniform, in all essentials, in their 

 atlas and axis ; but their caudal vertebras differ just as much as the 

 structures they support differ, e.g., the " Rectrices," or tail quills, 

 that form their double, fan-shaped, third iving. 



The Cormorant puts its tail to a much greater variety of uses than 

 the Swan ; the component vertebrae of the former are stronger and 

 have much larger intercentra to serve as levers to the depressors of 

 the tail. 



There are two movable caudals between the post- ilia in the Cormo- 

 rant, and the second of these has a seed-like intercentrum that lies 

 below and between the second caudal articulation. The next is much 

 larger, and the rest are as long and twice as broad as the neural spines 

 of the same vertebrae, and are ankylosed to the hinder bone; they 

 lie well under the one in front, and form the lower third of the pro- 

 ccelous joint. The last or compound bone has four of these intercentra 

 fused together and to the imperfect vertebrae to which they belong ; 

 thus this bone has a dilated and dentate base, the fore-part of which 

 passes under three-fourths of the last simple vertebra, and is bilobate, 

 whilst those in front are clavate. In some birds these intercentra have 



