PEOFESSOE OWEN ON THE GENUS DINOENIS. 149 



apophyses, homotypal with the prezygapophyses in the succeeding vertebrse. 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 a. 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 L. rohistus 

 (loc. cit.). 



Assuming the atlas of Struthio camelus i 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 Linornis 

 maximus. In the Ostrich the antarticular vacuity (not marked in Mivart's figures), 

 answering to a 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. 3, t, t) ; the hremal surface of the hypapophysis is produced downward 

 and backward into the (iuasi-\i^m?i\ spine, hj (Mivart, figs. 2-7) ; this is not present 

 in iJinornis, but is replaced by a pair of low tuberosities, fig. 2, hi/ (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, t, t) form part. The postzyga- 

 pophysial facets (/, z, 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 ^((asi-centrum; the processes, t, t, in fig. 2, 

 are not developed ; the preaxial cup has a wider upper emargination. 



The length of the axis in Dinornis maximus (fig. 4) is about four times that of the 



' Trans. Zool. Soc. vol. viii. p. 388, figs. 2-6. 



■ Iljid. Tol. iii. (1S42), pis. xxxiv. and xxsv. fig. 2, a**. 



^ Ibid. vol. iv. (1856), p. 162. 



t2 



