1888.] 



On the Vertebral Chain of Birds. 



471 



additional upper pair of confluent pre-zygapophyses form what Owen 

 calls a " zygosphene ;" this fits into a double cavity — the " zygantrum." 



Now the articulation of the opisthocoelous dorsals of the birds thus 

 mentioned is a complication of the articulation of centrum with 

 centrum, and not any special modification, in their case, of the neural 

 arch from which the zygapophyses spring. 



In Reptiles, as far as I can see, whether existing, or otherwise, there 

 is nothing like what I am about to describe ; if any Palaeontologist 

 will show me a similar structure I shall be most glad to know of it. 

 Such a fact would tell us how carefully these highly metamorphosed 

 types, the Birds, have kept along Reptilian Hues ; if not, if no such 

 structure as this, any more than the cylindroidal articulation, is ever 

 seen in Reptiles, then we have another instance of the manner in 

 which the Birds have proceeded beyond the excellencies of their 

 progenitors. 



The greatest perfection of this complex opisthoccelian articulation 

 of the dorsal vertebras is best seen in some remarkable Charadrian 

 birds ; three of which are Neotropical, whilst one is found in 

 Kerguelen's Island ; I refer to Ghionis, Attagis, and Thinocorus. 



In Attagis gayi, a Neotropical bird of the Plover family, stouter 

 than a Lapwing, but about the same size, a nearly extinct type, and 

 very archaic, I find the best instance of this Ornithic modification of 

 the opisthoccelian articulation of the dorsal vertebrae. On the hind 

 face of the centrum the cup in its fresh state is heart-shaped ; it is 

 half a long ellipse, with its upper edge gently emarginate. There is a 

 strong annular " meniscus," 1'5 mm. deep below, and 0"6 mm. wide for 

 the rest of its extent. It is a very solid fibro-cartilage, except for a 

 small extent above, where it is finished by a ligamentous part. When 

 this meniscus, which partly divides the joint cavity into two spaces, 

 is removed, the hollow cartilaginous tract is seen to be in three parts ; 

 below, a semicircular hollow, marked in its middle by the notochordal 

 "suspensory ligament," and above, on each side, a fiat ear-shaped 

 additional facet. These two facets look equally downwards and back- 

 wards, and they lie obliquely on a similar pair 'of facets over the ball 

 on the fore-end of the centrum of the next vertebra which looks up- 

 wards and forwards. These well-fitting oblique facets, fore and aft, 

 are, indeed, additional zygapophyses, arising not from the neural arch, 

 but from the centrum; and they check the movement of the cup-and-ball 

 joint. For a bird needs not only a very long and absolutely ankylosed 

 sacrum, it must also have a very strong dorsal series ; not unfre- 

 quently all but the last of this series are also ankylosed together ; 

 this only takes place in birds which have their dorsals cylindroidal. 



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