PEOFESSOE OWEN ON THE GENUS DINOENIS. 237 



given of the atlas of Dinornis rohustns, of the natural size. In this species the greatest 

 length of the vertebra, at the base of the neural arch, is 15 millim., the greatest breadth 

 at the same part is 30 millim., the height of the vertebra is the same ; the diameter of 

 the cup for the occipital condyle is 15 millim. In Dinornis parvus the corresponding 

 admeasurements are 10 millim., 20 millim., and 10 millim. respectively (PI. LI. figs. 2, 

 3,4). 



The proper centrum (ib. figs. 5, 6, 7, c') has coalesced with that of the second 

 vertebra (ib. c'), its anterior convexity forming the ' odontoid process,' and projecting 

 into the canal formed by the confiuent hypapophysis and neural arch of the atlas. The 

 hypapophysis (ib. fig. 2, hy) is, as usual, wedge-shaped, with the base downward, 

 as in Ichthyosaurus ; the apex is truncate, and deeply emarginate, to receive the odontoid 

 process. At the back part of the transversely extended and flattened base (ib. fig. 4) is 

 a feeble rudiment of the more developed process in Dinornis rohustus {loc. cit. pi. liii. 

 fig. 6, ky). The postero-lateral extensions give confluent attachments to the slender 

 columnar processes of the neurapophyses (ib. fig. 2, n) which bound outwardly the 

 large vertebrarterial foramina (ib. figs. 3, 4, v). The fore facet of the hypapophysis 

 is deeply cupped (ib. fig. 3) for articulation with the occipital condyle. The hind 

 facet (fig. 4, hy) gives a smooth, almost flat transverse surface to the corresponding 

 anterior facet of that of the axis (ib. fig. 8, c^). 



The base of each neurapophysis of the atlas has coalesced with the hypapophysis, 

 and there sends mesiad a short pointed process, tending to circumscribe the atlantal 

 cup below and the wide neural cavity above. Each neurapophysis then curves 

 upward, outward, and inward, coalescing with its fellow above the neural canal {n'), 

 over which it arches. From the hind part of the base of each neurapophysis extends 

 a short strong process, developing a tuberosity externally (z') and a flat, articular, 

 subcircular surface raesially, which joins the prezygapophysis of the axis. The roof 

 of the neural arch is a broad bridge-like plate of bone, without trace of neural spine. 

 In this respect it resembles that part in Dinornis inaximus, but difiers in shape and 

 extent. The mid line of the bridge in D. maximus equals one half of the transverse 

 extent ; in D. parvus it equals one third. The fore border of the bridge is straight in 

 D. parvus, but is convex in D. maximus. The hind angles supporting the postzyga- 

 pophysial surfaces {z) are relatively more produced in D. maximus, the upper surface 

 of the atlas presenting a horse-shoe figure. 



In Dinornis robustus the marked difference from both the above species is seen in 

 the non-union of the sides of the neural arch, leaving a longitudinal fissure at the 

 upper mid line {loc. cit. pi. liii. figs. 4, 5, c) ; it is possible that the specimen yielding 

 those figures may have come from an immature individual. The more complete ossifi- 

 cation of the vertebra in D. parvus is one of many similar evidences of the maturity of 

 the bird yielding the subject of the present description. 



