PROFESSOR OWEN ON THE GENUS DINORNIS. 149 
apophyses, homotypal with the prezygapophyses in the succeeding vertebre. ‘The 
vacuity is progressively encroached upon by the growth of the prezygapophyses, and, in 
the atlas of the aged individval of Dinornis maximus (fig. 3), it is reduced to the 
chink @ The neurapophyses have met and coalesced above the neural canal, which 
was not the case in the atlas of the younger, but full-grown, subject of D. robustus 
(doc. cit.). 
Assuming the atlas of Struthio camelus! to have been from a full-grown and mature 
individual, a similar confluence of the neurapophyses having taken place, the following 
differences are chiefly notable between it and the corresponding vertebra of Dinornis 
maximus. In the Ostrich the antarticular vacuity (not marked in Mivart’s figures), 
answering to @ in fig. 3, remains much more widely open; the hypapophysial surface 
(ac) is less deep in proportion to its breadth; its lower border has not the pair of low 
tubercles (fig. 5, ¢, ¢); the heemal surface of the hypapophysis is produced downward 
and backward into the guasi-hemal spine, hy (Mivart, figs. 2-7); this is not present 
in Dinornis, but is replaced by a pair of low tuberosities, fig. 2, hy (which productions 
served for the attachment of the ‘ longus colli’?), as in Apteryx, but are variable. The 
difference between Dinornis and Struthio in the relative size of the vertebrarterial 
foramina v is well marked ; the larger size of the canal in Dinornis relates to its better- 
developed brain: the roof of the neural canal is relatively less extended from before 
backward in Struthio; it is convex, rough or irregular in surface, with a feeble indi- 
cation of a medial ridge at the fore part in Dinornis (fig. 1, n), and with a hyper- 
apophysis (ib. hp) as a low tuberosity above each postzygapophysis. 
The postaxial articular surface presents, in Dinornis, a subquadrate convexity, and is 
not flat transversely in the present species or individual: the upper shortest border is 
moderately concave ; the lower longest border is framed, as it were, by a backwardly 
extended ridge, of which the pair of tubercles (fig. 3, ¢, ¢) form part. The postzyga- 
pophysial facets (z’, 2’, fig. 2) look more obliquely backward than in Struthio, where 
their aspect is almost wholly inward or ‘ mediad.’ 
In the general though slight convexity of the postaxial articular surface of the 
atlantal hypapophysis, in the slenderness of the pardiapophysial bar (fig. 3, pd), 
defining outwardly the vertebrarterial canal, and in the parial disposition of the 
hypapophysial tubercles, the atlas of Dinornis elephantopus* in the main agrees with 
that of D. robustus and D. maximus. 
In Struthio the neural arch has a less relative antero-posterior breadth, and the 
same proportional difference prevails in the guasi-centrum ; the processes, f, ¢, in fig. 2, 
are not developed; the preaxial cup has a wider upper emargination. 
The length of the axis in Dinornis maxinws (fig. 4) is about four times that of the 
Trans. Zool. Soc. vol. viii. p. 388, figs. 2-6. 
> Tbid. vol. iii. (1842), pls. xxxiv. and xxxv. fig, 2, a**. 
» Ibid. vol. iv. (1856), p. 162. 
