PEOFESSOE OWEN OX THE GENUS DINOENIS. 165 



The centrum and neural arch show no notable differences from those of the sixteenth 

 vertebra. The pleurapophysis, with a similar double articulation, has increased in iize 

 and conspicuously in length ; it also supports an ' epipleural appendage,' which is broad 

 and slightly curved upward, where it overlaps the succeeding rib. The hsemapophysis 

 is ossified and synovially articulated with the pleurapophysis above and with the haemal 

 spine (' sternum ') below. Towards its h;iemal end it expands and develops a tubercle. 



The size of the dorsals in D. maximus increases slightly as they recede, and chiefly 

 in breadth, by the outgrowth of the diapophyses, accompanied by a greater size of 

 the rib and greater length and divergence of its capitular and tubercular processes. 

 In the present vertebra (third dorsal) the base of the hypapophysis, though shorter 

 than in the second dorsal, occupies a greater extent of the haemal surface of the 

 centrum than in the third dorsal of Strut kio. Not more of the fore part of the neural 

 surface of the centrum is exposed than in the antecedent dorsals and terminal cervical 

 vertebra. The postaxial surface continues to be narrow in proportion to its height ; 

 but its transverse convexity increases, and is relatively greater than in Struthio. The 

 transverse concavity of the preaxial surface has also increased ; it is still convex ver- 

 tically along its middle third. In a homologous vertebra of the present species of 

 Binornis I have noted a variety in the hypapophysis in the interruption of its basal 

 extent producing a small g^iiasi second hypapophysis near the postaxial surface. 



The pleurapophysis, with a slight increase of length, and of that of its appendage, is 

 as in the second dorsal. The hsemapophysis (' sternal rib ') articulates by a trans- 

 versely extended bitubercular end with the sternum. 



The nineteenth vertebra (fourth dorsal, figs. 27-29), corresponding with the first of 

 those having their pleurapophyses free and articulating with their hffimapophyses in 

 Struthio (figures 47, 48, ' Mivart,' p. 413), has the centrum less cuneiform in transverse 

 section, the sides converging, with a certain convexity, hsemad to a low and short ridge or 

 keel, produced and thickened anteriorly, near the preaxial surface (fig. 27, hy). 



Prof. Mivart reckons the dorsal series as commencing with the vertebra thus typi- 

 cally complete in regard to its hsemal arch. I prefer to retain the character of a free 

 pleurapophysis as denoting the present class of axial segments. 



Thus the nineteenth vertebra in Dinornis, or fourth of the dorsal series, answers to 

 the twenty-first in Struthio, which is the fourth supporting a free pleurapophysis 

 (vertebral rib), and the first in which this element articulates with its hsemapophysis 

 (sternal rib). The hsemapophysis of the twentieth vertebra in Struthio is developed, 

 but is articulated only with its spine (sternum) and does not join by its opposite end 

 the pleurapophysis. Such condition I have not yet seen in any species of Dinornis. 



With respect to the twenty-first vertebra in Struthio, Mivart remarks, that " it is so 

 much like the twentieth that little need be said in its description" (p. 413). My 

 figure 28 may therefore be contrasted with figure 46 in Mivart's monograph (p. 411, 

 loc. cit.) for illustrations of the differential characters in question. 



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