439

409

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 size
and conspicuously in length ; it also supports an 'elipleural appendage,' which is broad
and slightly curved upward, where it overlaps the succeeding rib. The haemapophysis
is ossified and synovially articulated with the pleurapophysis above and with the haemal
spine ('sternum') below. Towards its haemal 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 acompanied 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 Struthio. 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
Dinornis I have noted a variety in the hypapophysis in the interruption of its basal
extent producing a small quasi 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 haemapophysis ('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 haemapophyses in
Struthio (figures 47, 48, 'Mivart,' p. 413), has the centrum less cuneiform in transverse
section, the sides converging, with a certain convexity, haemad 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 haemal 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 haemapophysis
(sternal rib). The haemapophysis 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.
3 S