1888.] 



On the Vertebral Chain of Birds. 



469 



billed Dodmastes ensifer* I find only three. In the lesser Passerines, 

 as a rale, only one pair of sacral ribs are developed. 



On the Articulation of the Vertebral in Birds. 



Special modification of the vertebral chain takes place to a greater 

 extent in birds than in any other of the Vertebrata. Even in the 

 intensely modified vertebrae of Serpents, with their zygosphene and 

 zygantrum, we still have merely the " procoelous " articulation of the 

 centra. 



But in birds, as soon as the short-tailed forms appear, we have as in 

 Marsh's gigantic Hesperornis, or feeble-winged Colymbine Grebe with 

 plenrodont teeth — the highest known form of the vertebral articula- 

 tion, namely, the " cylindroidal " or " heteroccelous." This most 

 accurate mode of locking the vertebral segments together, in which 

 the centra viewed from below seem to be proccelous, but seen endwise 

 or laterally are opisthocoelous, is peculiar, as far as I know, to birds, 

 and apparently was not always, or from the beginning, present in 

 them. This seems to be shown by the fact that the other of Marsh's 

 toothed birds, namely, Ichthyornis, has for the most part " amphi- 

 coelous " vertebrae, only one or two joints at the upper part of the neck 

 showing the cylindroidal articulation, and that imperfectly. 



Now this is a most puzzling fact in Palaeontology, for Ichthyornis is 

 a Carinafce bird, and as far as I can see, is the parent form of the Gulls 

 (Laridae), although it possesses thecodont teeth in its long jaws. 

 Everyone knows that the Loons and Grebes (Colymbus, Podilymbus, 

 and Podiceps') are " Pygopods," and rather of a low type, but the 

 Gulls are amongst the noblest and most intelligent of the Palmipeds, 

 and are semi-altricial in their breeding". 



o 



Now it is a fact that modern Grebes and Loons disagree with the 

 other Pygopods in having all their presacral vertebrae cylindroidal, 

 whilst the Alcidae and the Penguins (Spheniscidae) and Gulls have 

 their dorsal vertebrae opisthocoelous. More than this, by careful 

 examination of the fore end of the first sacral {dor so- sacral) vertebra 

 in the lesser Gulls (Lams canus, L. ridibundus, L. tridactylus) , I find 

 that this is nofc a ball to fit accurately into the cup of the last free 

 dorsal, but that its facet is sinuous, and does leave some space inside 

 the joint. 



Hence I infer, cautiously, but with some considerable degree of con- 

 fidence, that the modern Gulls have not quite perfected even the lower 

 or " opisthocoelous " form of articulation of the vertebral centra for 

 all their dorsals. 



I call the opisthocoelous mode of articulation loiver, because it cer- 

 tainly comes short in type of the cylindroidal, and is I believe the 



* In that bird, which is much smaller than Patagona,t\\e whole skull is h\ inches 

 long (137 mm.), and the rest of the axis 2£ inches (56 mm). 



VOL. XLIII. 2 L 



