1888.] 



On the Vertebral Chain of Birds. 



473 



In Eupodotis the proccelous facet of the atlas is a crescent with 

 horns approximating, and between these the odontoid process, or true 

 atlantal centrum, appears; it is embraced by these "horns," and, as 

 in the other type, is tied to the basi-occipital by the suspensory liga- 

 ment. In this bird, contrary to the rule, the atlas does not grow 

 under the axis, and the joint between them is almost proccelous ; 

 and in this and still more in some other Precoces, the occipital arti- 

 culation is transversely enlarged, i.e., shows signs of being double, 

 as in Amphibia, the notochordal dimple answering to the wide inter- 

 space between the condyles in these forms. 



I shall explain these things more perfectly when I come to the 

 " intercentra." 



The imperfect joints are those of the sacrum and the coccygeal bones. 

 The long general sacrum of a bird does not correspond to the special 

 sacrum of a Reptile or a Mammal, and' in the dorsal region of this 

 long series the articulations are, at first, like those of the free dorsals 

 in front of them ; i.e., they are cylindroidal or opisthoccelous, as the 

 case may be. But as we approach the true sacral region, between the 

 acetabula, the faces of the centra are roughly flat, and the centra 

 themselves are transverse subcrescentic blocks, with all the inter- 

 central structures aborted. 



The same thing takes place in the ploughshare or coccygeal bone, 

 which finishes the chain by a series of from four to six, more and more 

 imperfect, segments, from which, for a time, in the embryo the noto- 

 chord projects, uncovered, behind. 



There are other ankylosed parts of the vertebral chain besides the 

 sacrum and the coccygeal bone ; in these the parts are normal at first, 

 becoming afterwards fused together. It is very common for the last 

 cervical (whose free rib does not unite by a short piece with the 

 sternum) to be fused with the dorsals — all but the last, which remains 

 free, as in Falcons, Pigeons, Fowls, &c. The same thing* takes place 

 in many of the Crane family, but generally with fewer bones. In the 

 Hornbills (Buceridas) the atlas and axis become ankylosed. In some 

 other Al trices we have there found that which is normally the last free 

 dorsal fused with the first of the dorso-sacral series ; and in others 

 the first dorsal sacral, covered by the iliac bones, remains free ; this 

 is, however, a very irregular modification, and is sometimes due to 

 old age in one case, and to a somewhat immature condition in the 

 other. 



In the present paper I cannot go into details as to the various modi- 

 fications of the neural arches, with their zygapophyses and spines, nor 

 describe the various outgrowths below that arise from the centra. 

 But there are distinct parts of the vertebra that must be mentioned ; 

 these are the " intercentra " and ribs. 



